Undefended Piece
An undefended (loose) piece is a knight, bishop, rook or queen that no friendly piece protects. "Loose Pieces Drop Off" — LPDO: when a piece is loose and one of your pieces attacks it, you simply take it for free, because there is no recapture. Spotting loose pieces, yours and your opponent's, is the single most common source of tactics.
Source for further study: Nunn, Secrets of Practical Chess (LPDO).