Definition: Succinctly, a "gimmie" is a putt so short a golfer considers it unmissable, so just picks it up and counts it as holed. To be more precise: A gimmie putt is one that a player requests be conceded by another player, allowing the first player to pick up and move on as if the putt had been holed.
The word derives from "give me," as in, "Will you give me that one?" Many recreational golfers play using gimmies for any short putt. The unofficial rule is "inside the leather" - that is, if a ball is closer to the hole than the distance from your putter head to putter grip when laid flat on the green, it's a gimmie and you can pick up.
The key word there is "unofficial." Because gimmies are never allowed under the official rules. In match play, an opponent can concede a putt of his or her own volition (without being asked), but the rules never allow gimmies.
And some golfers who use gimmies when playing with buddies abuse the practice by imagining longer and longer putts as ones they will, of course, make, so why not take a gimmie?
Gimmies do speed up play on the greens, and if your recreational group agrees among its members to use them, go for it. Just remember: You cannot, under any circumstances, use gimmies in any competition played under the official Rules of Golf (including rounds posted for handicaps). Also note that many golf instructors consider gimmies counterproductive to good putting: You need to see and hear that ball dropping into the cup to build confidence. Using gimmies undercuts that.
Alternate Spellings: Gimme