From the racing
"Tack, Starboard or Port A boat is on the tack, starboard or port,
corresponding to her windward side
"Leeward and Windward A boat’s leeward side is the side that is or, when
she is head
, was away from the wind
. However, when sailing by the
lee or directly downwind, her leeward side is the side on which her mainsail
. 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."
From the Colregs:
"For the purpose of this Rule
the windward side shall be deemed to be the side opposite to that on which the mainsail is carried
or, in the case of a square-rigged vessel, the side opposite to that on which the largest fore-and-aft sail is carried."