The dissipation of sound energy on striking a physical surface.
 The portion of sound energy not absorbed is either bounced back as reflected sound or transmitted through the material. 

Rooms with more reflective surfaces (little absorption) will result in longer reverb times and are referred to as "live" or "wet", while rooms with very absorptive surfaces will result in relatively short reverberation times and are referred to as "dead" or "dry". 

In recording situations in a room with poor absorption this can lead to "muddiness" of the resulting recorded sound. The solution to recording in an overly "live" room is to place the microphone (microphone placement) closely to the sound source. This will result in a much greater ratio of wanted sound to unwanted ambient noise effectively masking the acoustic quality of the surrounding space.  Sometimes close micing is impractical when recording complex sounding objects such as large instruments. In this case adjustments to the absorptive characteristics of the room itself can be made by covering the reflective surfaces with absorptive material such as carpet, cardboard, or acoustic baffling.