In the card game "contract bridge", a splinter bid is a convention whereby a double jump response in a side-suit indicates excellent support (at least four cards), a singleton or void in that side-suit (but preferably not the ace or king), and at least game-going strength.
[1] Some partnerships agree that the maximum strength can be only that necessary to reach a game contract; stronger holdings with major suit support instead might temporize with a Jacoby 2NT bid.
Different bidding systems may specify different strengths for responder's hand, e.g. 10-12 high card points (HCP), 10-14 or 11-15, for a splinter.
Although they consume bidding space, splinter bids are very descriptive as they help partner to reevaluate his/her hand: soft honors (a king, queen or jack) in the splinter suit lose value, while honors in the other three suits gain value.
The idea is that partner can easily tell if he has wasted values in the splinter suit; for example, Axxx is ideal whereas KJ9x is almost worthless.