1 That part of the side of a ship which is above the main wales. The term referred particularly to square-rigged sailing warships, where the main wales ran level with the bottom of the upper deck gunports. In its modern meaning it usually refers to that portion of the ship's side which rises above the upper deck though the term is often loosely used to refer to the upper deck itself. For example ‘I'm going topsides’ means ‘I'm going on the upper deck.’
2 The sides of yachts, above the boottop, are also known as topsides.