Official definitions are listed in the ISAF Racing Rules of Sailing Definitions.

Abaft--Toward the rear (stern) of the boat. Behind.
Abandon: A race that a race committee or protest committee abandons is void but may be resailed.
AbeamAt right angles to the keel of the boat, but not on the boat. Or, At right angles to the yachts midships.
Above Deck: On the deck (not over it see ALOFT)
Advantage Line: An imaginary line drawn perpendicular to the true wind direction across the bow of the leading yacht. Wind shifts may cause the advantage line to swap between yachts.
Aft: Toward the stern of the boat.
Alee: Away from the direction of the wind. Opposite of windward.
AloftAbove the deck of the boat.
AmidshipsIn or toward the center of the boat.
Apparent Wind: The wind one feels when moving. A combination of true wind speed and that created by the moving yacht.
Aspect Ratio: The height of a sail or keel divided by its width.
Astern: In back of the boat, opposite of ahead.
AthwartshipsAt right angles to the centerline of the boat; rowboat seats are generally athwart ships.
Attached Flow: Flow of a fluid (air or water) that generally follows the contours of a foil, (ie; sails, keels, rudders and other appendages).
Backstay: A wire support for the mast, usually running from the stern to the head of the mast.
Backwater: To row in reverse, to slow or stop a boat, or to move the boat backwards.
BackwindThe bubble in the mainsail luff caused by air flowing off the jib.
Ballast WeightUsually metal, placed low in a boat to provide stability.
Barber Hauler: A line attached to the jib or jib sheet, used to adjust the angle of sheeting by pulling the sheet toward the centerline of the boat.
BattensFlexible strips of wood or plastic, most commonly used in the mainsail to support the aft portion, or roach, so that it will not curl.
Batten Pockets: Pockets sewn into the trailing edge of the sail to hold the battens.
Beam: The width of the hull, often taken to mean the maximum width.
Beam Reach: Sailing with the wind directly abeam.  (See Point of Sail )
Bear AwayBear Off: To steer away from the wind. (See Point of Sail )
Bearing: The direction of an object expressed either as a true bearing as shown on the chart, or as a bearing relative to the heading of the boat.
BeatTo sail to windward, also known as sailing close-hauled. (See Point of Sail )
Bilge: The area in the lower part of the hull where water collects, also the part of the hull where the bottom turns into the side, the "turn of the bilge".
Blanketing: A tactical maneuver in which one yacht position is itself to obstruct or disturb its competitor's wind.
Block: Known on land as a pulley. IE. block and tackle, turning block, etc.
BoomThe spar to which the foot of the mainsail is attached, See also Jib boom, Jib club
Boom Vang: A system used to hold the boom down, particularly when boat is sailing downwind, so that the mainsail area facing the wind is kept to a maximum. Frequently extends from the boom to a location near the base of the mast. Usually tackle- or lever-operated.
Boot Top: A painted line that indicates the designed waterline.
Bow: Front end of the boat.
Broad ReachSailing so that the wind is behind the beam.  (See Point of Sail )
By the Lee: Sailing so that the wind is on the same side as where the main is carried. When running, this could happen if there is a wind shift to the side of the boat where the main is. Sailing by the lee is discouraged because it could result in an accidental Gybe.  (See Point of Sail )
Chainplate: Fitting that connects shrouds (or side stays) to hull.
Clear Astern and Clear Ahead; Overlap: One boat is clear astern of another when her hull and equipment in normal position are behind a line abeam from the aftermost point of the other boat's hull and equipment in normal position. The other boat is clear ahead. They overlap when neither is clear astern or when a boat between them overlaps both. These terms do not apply to boats on opposite tacks unless rule 18 applies.
Clew: Bottom back corner of sail.
Close Hauled: Sailing as close to the wind as possible. (pointing) (See Point of Sail )
Close Reach: Sailing between close hauled and beam reach. (See Point of Sail )
Cringle: Metal reinforcement ring.
Cunningham: The cringle (grommet) on the luff (forward edge) of the sail used to achieve luff tension for draft control. (sail shaping)
Downhaul: Line used to tighten or tension the luff (forward edge) of the sail.
Ease: To let out... as in ease a sheet. (line)Fairleads: Eyes or blocks that guide lines in a desired direction. Usually they are used for mainsheets and jibsheets on R/C Sailboats.
Foot: Bottom edge of sail.
Forestay: The wire (cable) that supports the mast from the bow or tip of jib club and prevents the top of the mast from moving aft.
Finish: A boat finishes when any part of her hull or equipment in normal position, crosses the finishing line in the direction of the course from the last mark either for the first time or, if she takes a penalty, after complying with rule 31.2 or rule 44.2. or, under rule 28.1, after correcting and error made at the finishing line.
Gooseneck: The fitting that connects the boom to the mast. It works like a swivel allowing the boom to move up and down and swing from side to side.
Gybing: Turning the stern of the boat through the eye of the wind.
Halyards: Lines or wire rope used to hoist (or tie) the sails (to the top) of the mast.
Head Up: Turning the bow of the boat towards the eye of the wind.
Head: Top corner of sail. (Where halyard connects to sail)
Header: Change in wind direction towards the bow of the boat.
Helms-A-Lee: Verbal notification that the tiller has been put to leeward to cause the boat to come about. (tacking).
Interested Party: A person who may gain or lose as a result of a protest committee's decision, or who has a close personal interest in the decision.
In Irons: The condition when the boat is pointed directly into the wind without forward movement or the ability to steer.
Jib BoomJib ClubThe spar to which the foot of the jib is attached, usually only seen on R/C Sailboats.  See boom also.
Keel: The weighted fin at the bottom of the boat that keeps the boat from slipping sideways through the water.
Keep Clear: One boat keeps clear of another if the other can sail her course with no need to take avoiding action and, when the boats are overlapped on the same tack, if the leeward boat could change course without immediately making contact with the windward boat.
Lee Helm: The tendency of a sailboat to head away from the wind if the helm is released.
Leech: Back edge of sail or diagonal measurement from Head to Clew
Leeward and Windward: A boat's leeward side is the side that is or, when she is head to wind, was away from the wind. However, when sailing by the lee or directly downwind, her leeward side is the side on which her mainsail lies. The other side is her windward side. When two boats on the same tack overlap, the one on the leeward side of the other is the leeward boat. The other is the windward boat.
Leeward: The direction to which the wind is going.
Lift: Change in wind direction towards the stern of the boat. A header for a boat on port tack is a lift for a boat on starboard tack.
Luff: Forward edge of sail.
Luffing: The fluttering of a sail when a boat is pointed too close to the wind or the sail is eased out too far.
Mark: An object the sailing instructions require a boat to pass on a specified side, excluding its anchor line and objects attached temporarily or accidentally.
Mast: The large vertical spar that supports the sail and boom.
NOR: Notice of Race - the official announcement of a regatta and the document that establishes many aspects of the regatta.
Obstruction: An object that a boat could not pass without changing course substantially, if she were sailing directly towards it and one of her hull lengths from it. An object that can be safely passed on only one side and an area so designated by the sailing instructions are also obstructions. However, a boat racing is not an obstruction to other boats unless they are required to keep clear of her or give her room.
Overlap See Clear Astern and Clear Ahead Overlap.
Party A party to a hearing: a protestor; a protestee; a boat requesting redress; any other boat or a competitor liable to be penalized, including under rule 69.1; a race committee in a hearing under rule 62.1(a).
PHRF Performance Handicap Racing Fleet, A system by which dislike boats may be raced against each other.
Points of Sail The recognized terms for which direction you are sailing relative to the wind
Close HauledClose ReachBeam ReachBroad ReachRunningBy the Lee
See diagram below
Postpone: A postponed race is delayed before its scheduled start but may be started or abandoned later.
Proper Course: A course a boat would sail to finish as soon as possible in the absence of the other boats referred to in the rule using the term. A boat has no proper course before her starting signal.
Protest:  An allegation by a boat, a race committee or a protest committee that a boat has broken a rule.
Racing Rules of Sailing: (RRS) Formerly the International Yacht Racing Rules (IYRR), the officially recognized rules for racing sailing vessels. These are approved and published by the International Sailing Federation (ISAF) and are available from US Sailing (USSA) and most sailing related stores in the United States.
Racing:  A boat is racing from her preparatory signal until she finishes and clears the finishing line and marks or retires, or until the race committee signals a general recall, postponement, or abandonment.
Room:  The space a boat needs in the existing conditions while maneuvering promptly in a seamanlike way.
(a) The rules in the RRS book, including the Definitions, Race Signals, Introduction, preambles, and the rules of an appendix when it applies, but not titles;
(b) the prescriptions of a national authority, when they apply;
(c) the class rules except any that conflict with the rules in this book;
(d) the notice of race
(e) the sailing instructions;
(f) any other documents governing the event.
Running: Sailing so that the wind is directly astern. The jib and main sails will be on opposite sides. (wing and wing). (See Points of Sail)
SI: Sailing Instructions - The Official instructions regarding how racing will be conducted, see example SI by clicking here.
Start: A boat starts when after her starting signal any part of her hull, crew or equipment first crosses the starting line and she has complied with rule 29.1 and rule 30.1 if it applies.
Tack, Starboard or Port: A boat is on the tack, starboard or port, corresponding to her windward side. (See Points of Sail)
Two-Length Zone: The area around a mark or obstruction within a distance of two hull lengths of the boat nearer to it.
Windward See Leeward and Windward .