Idle animations refer to animations within video games that occur when the player character does not do any action (hence being idle). These sorts of animations are largely popular with action games, where players may rarely be idle and thus serve as easter eggs. One of the first games ever to introduce idle animations on a mainstream basis was Maziacs; the sprite taps his feet, blinks, and sits down. This trend spread to many other action genre games, becoming more complex as allowed by the advancing hardware. For example, in Donkey Kong Country 2: Diddy's Kong Quest, Diddy Kong juggles a few balls if the player offers no controller input for a few seconds.
Idle animations in modern 3D games are used to keep them realistic. In general, games meant for younger audiences are far more likely to have complex or humorous idle animations, while games for older people tend to have more basic idle animations. For example in Super Mario 64, Mario will occasionally look around, and eventually fall asleep, while in others, steady breathing suffices as an idle animation.