VPIP

(Voluntarily Put In Pot)
stats

% of hands a player voluntarily enters (excludes blind posts).

Solid 6-max regs: 20–25%. Loose-passive fish: 35%+. Nits: under 15%.

See also

Fold-to-CBet
% of times a player folds to a continuation bet.
Aggression Factor
(Bets + Raises) / Calls — how aggressively a player plays.
WTSD
% of pots a player sees showdown after the flop.