Here are some definitions from the new Racing Rules of Sailing. I have picked just a few choice ones that everyone should thoroughly understand. Understanding the rules requires knowledge of the definitions. All italicized words in the RRS are listed in the Definitions.
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 can change course in both directions without immediately making contact with the windward boat.
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.
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.
Room - The space a boat needs in the existing conditions while maneuvering promptly in a seamanlike way.