1. The technique of remote sensing in which an instrument sends acoustic waves vertically and receives reflections from atmospheric features such as inversions or turbulent layers. 2. A representation of the vertical profile of one or more of the state variables of a fluid body (typically the ocean or atmosphere) deduced from variations in reflected acoustic energy. 3. A particular measurement of depth of water below an instrument (either at the surface or at some moored depth) computed from the travel time of an acoustic pulse emitted by the acoustic sounder.