Idiot end or Ignorant End - In flop games, a player drawing to, or even flopping, a straight with undercards to the flop has the idiot end of it. A player with 8-9 betting on a flop of A-T-J puts himself at great risk, because many of the cards that complete his straight give credible opponents higher ones.

Image  - The way other players see you.

Immortal - Unbeatable; often said of a hand that a player knows cannot be beaten under the circumstances of play.

Implied Odds  - The odds you calculate, considering the assumed result of betting for the remainder of the hand.

Implied odds - The ratio between the amount of money a player is expected to win on a particular hand and the amount that he is required to put in the pot to stay in the game.

Improve - Strengthen one's hand.

Insurance - A business deal in which players agree to split or reduce a pot (roughly in proportion to the chances of each of them winning) with more cards to come rather than playing out the hand, or else a deal where one player makes a side bet against himself with a third party to hedge against a large loss. This is usually done when one player is all in.

In the bushes, in the weeds - A player sandbagging is said to be in the bushes during the time he is quietly checking and calling while others bet aggressively.

In The Dark - Betting without knowledge of either your hole cards or before the next card is dealt.

In the middle - In a game with multiple blinds, an incoming player may sometimes be allowed to post the blinds in the middle (that is, out of their normal order) rather than having to wait for them to pass.
A player being whipsawed is said to be in the middle.

In the money - In tournament play, to reach a place in which money is awarded.

Inside - Pertaining to an inside straight. To catch inside means to make an inside straight. An inside straight card is often called a gutshot.

Inside straight draw - Similar to a straight draw, except you are trying to catch an inside straight card. For example, if you start in hold'em with Ts Js and the flop is 8h 7d 2d, you have an inside straight draw.

Inside Straight  - Four cards that require another between the top and the bottom card to complete a straight. Players who catch this card make an Inside Straight.

In turn - A player, or an action, is said to be in turn if that player is expected to act next under the rules. Jerry said "check" while he was in turn, so he's not allowed to raise.

Irregular declaration - An action taken by a player in turn that is not a straightforward declaration of intent, but that is reasonably interpreted as an action by other players, such as rapping the table to signify "check", or pointing a thumb up to signify raise. House rules or dealer discretion may determine when such actions are meaningful and/or binding.

Irregularity - Any of a number of abnormal conditions in play, such as unexpectedly exposed cards, that may call for corrective action.

Isolation - To play aggressively in order to drive out all but one specific opponent who you believe to be weak. I re-raised to isolate Maria because she's been playing junk all day.

'I wait' - To check.