The word piece has three meanings, depending on the context:
a) It may mean any of the physical pieces of the set, including any of the pawns.
b) It may be used to exclude pawns, referring only to a Queen, Rook, Bishop, or Knight, and perhaps also the King.
c) It may refer only to a minor piece (a Bishop or Knight).
The context should make the intended meaning clear.
Source: "Wikipedia, The free Encyclopedia"