It’s any playable part of the body, so basically everything except the hands and arms.
Direct link to the specific Law.
A player is in an offside position if:
any part of the head, body or feet is in the opponents’ half (excluding the halfway line) and
any part of the head, body or feet is nearer to the opponents’ goal line than both the ball and the second-last opponent
The hands and arms of all players, including the goalkeepers, are not considered.