The betting unit of the later rounds of fixed limit, twice the small bet.
The turn and river in Hold'em and Omaha, 5th–7th street in Stud, after the second and third draws in Triple Draw. At $10/$20 it's 20. Bets double here, so this is the round where you really decide whether to continue. Different from the "big bet" in Big Bet Mix (the NL and PL games).