A stop segment is defined as a segment whose medial phase is characterized by a stricture of complete oral closure made by the active articulator against the passive articulator. This prevents the escape of air from the mouth, and if a simultaneous velic closure is maintained so that the air cannot escape through the nasal cavity either, then under conditions of positive egressive pressure, the intra-oral pressure (i.e. the pressure of the air within the vocal tract) will rise. These conditions constitute the minimum requirements for the specification of a pulmonic egressive oral stop segment.

When the oral closure is released in the offset phase of an oral stop of this sort, the compressed air escapes to the atmosphere with a small but audible explosion, sometimes referred to in the acoustic phonetic literature as the stop burst. In the case of a pulmonic ingressive airstream, the intra-oral pressure of the volume of air which is sealed from the outside atmosphere by the oral and velic closures drops. When the seal is released, the higher-pressure air of the outside atmosphere implodes in to fill the relative vacuum in the vocal tract.

Stops, like other segment-types, have three phases. These all relate to the oral closure that characterizes the medial phase. The onset phase, as the articulatory transition towards complete closure, can be called the closing phase. The medial phase of a stop segment (from the moment full closure begins to the moment it ends) can be referred to as the closed phase.