An extremely tight player.
Rule of thumb: VPIP 15% or less, entering only with strong hands. Adjustments: bluffs work, believe their raises and fold, no thin value, almost no implied odds for draws (they only have strong hands, so they won't pay you off), and bet small to take the pot when they check. Example. Against an early-seat open, 3-bet only KK+ and AK; against a CO, BTN, or SB open, 3-bet your baseline range as is (they fold to 3-bets a lot).
At the table
Players say things like "they're a nit, so a raise from them means about AA or KK," about someone who only plays strong hands.