VP$IP aka VPIP
"Voluntarily put money into the pot."
This generally refers to calling or making a pre-flop bet -- either calling or raising when not in the
blinds, or calling or reraising from the blinds.
It does not include if you are in the big blind and see the flop if it is not raised.
Often people will speak of VP$IP as a representation of how
tight a player is,
but this is not really accurate. It reflects how loose or tight a player has played.
Since in some games it is appropriate to play looser (or tighter) than others, comparing numbers from two different players playing in
different games will often be deceiving.