The ACT extension is given to files that are used to store digital audio material generated with Adaptive Differential Pulse Code Modulation. It is a lossy compression format used, for example, in voice recorders or MP3 players. This reduces the amount of memory required for storing audio data.

In the case of an analog data source (e.g. a human voice), the digital audio recording process must be preceded by sampling. Today, ACT files are rarely used and this format is considered obsolete.