M
Main Pot The center pot. Any other bets are placed in a side pot(s) and are contested among the remaining players. This occurs when a player(s) goes all-in.
Make To make the deck is to shuffle the deck.
Maniac A very aggressive player who plays a lot of hands.
Middle Pair In flop games, when a player makes a pair with one of his/her down cards and the middle card on the flop.
Middle Position Somewhere between the early and late positions on a round of betting (the fifth, sixth and seventh seats to the left of the button).
Muck To discard or throw away your hand. It's also a pile of cards that are no longer in play.
Minimum Buy-In The least amount you can start a game with.
Monster A very big hand. In a tournament, a player who begins to accumulate chips after having a small stack is considered to be a monster.

N
No Limit A game where players can bet as much as they like (as long as they have it in front of them) on any round of betting.
Nuts The best possible hand at any point of the game. A hand that cannot be beat.

O
Odds The probability of making a hand vs. the probability of not making a hand.
Offsuit Cards of a different suit.
Omaha A game in which each player is dealt four down cards with five community cards. To make your hand, you must play two cards from your hand and three from the board.
Open To make the first bet.
Open-ended Straight Four consecutive cards whereby one additional (consecutive) card is needed at either
end to make a straight.
Open Card A card that is dealt face-up.
Open Pair A pair that has been dealt face-up.
Option An option is a Live Blind made in the dark before the cards are dealt. If no one raises, the "option" player may raise the pot.
Out Button A disc placed in front of a player who wishes to sit out a hand(s) but remain in the game.