Adaptive audio is audio that changes according to the state of its playback environment. In many ways, it is like interactive audio in that it responds to a particular event. The difference is that instead of responding to feedback from the listener/player, the audio changes according to changes occurring within the game or playback environment. Say for instance that a game's musical score shifts keys with the rising and setting of the sun within the game world. The player isn't causing the sun to set; the game is. Therefore, the score "adapts" to changes happening within the game's world. A famous example of adaptive audio in games occurs in Tetris. The music plays at a specified tempo, but that tempo will double if the player is in danger of losing the game.
Avoiding repetition is an excellent first step in a strong audio implementation for games, as well as reintroducing music listeners to some potentially intriguing performance characteristics lost when listening to linear music. Continuing to focus on audio for games for a moment, audio content triggered out of context to game events is in many ways less desirable than simple repetition. For instance, using data compression and/or high-capacity media, a composer might be able to create hours and hours of underscore, but if this linear music is played with no regard to the state of the game or the current events in the game's plot, it could be misleading or distracting to the user. Adaptive audio (audio that changes according to changes in certain conditions) provides an important first step for creating interactivity in underscore.
Do not confuse adaptive audio with variability — if, for instance, 20 variations of a gunshot are created and a single one is randomly chosen when the player fires a gun, that is variable but not necessarily adaptive. If the gunshot's reverberation characteristics modulate as the player navigates various and differing environments, then we have adaptive sound design. Likewise, a game's score could have infinite variability, never playing the same way twice, but it becomes adaptive when the sultry slow jazz tune fades in over the rock theme during the player's introduction to the game's film noir femme fatale antagonist. Now contrast variability and adaptation with interactivity; a character that stops talking when shot by the player is an example of interactive audio.
Adaptation does not always need to mean "just-in-time" audio generation. While an underscore that instantly and obviously responds to game events might be appropriate for a children's animated title, this kind of music is often inappropriate for a first-person shooter, for instance. In such a style of game, where the breakneck pace of the action constantly modulates the game state, the user can begin to notice the interactivity of the score. This is dangerous, as the subtlety of the score is lost on the player, potentially damaging the designer's carefully crafted game experience. In a game like Halo, you do not want the player performing music via an exploit in the interactive music engine. For this reason, musical changes are often most satisfying when "immediate" changes are reserved for important game events and more subtle interactivity is used for other events and situations.