In consensual adult spanking, a session is something similar to a scene: a play that involves spanking, and optionally other activities such as roleplay, other punishments, showing love and tenderness, and/or sexual activities.
A session normally involves two people, a top and a bottom, but this concept can also be expanded to one top and two bottoms, or one bottom and two tops, or similar variants. Tops and bottoms can be of any gender, so a session can be M/F, F/M, F/F or M/M, etc.
A session is typically a unit that is not interrupted by other activities that are not part of the play. A session can take from a few minutes up to one or two hours (if periods of corner time, for example, are included as part of a session). An overnight session is an extended session in which the partners spend a whole evening and night together.
There can be several spankings in the same session, in different states of undress, in different positions, and with different implements. A session often follows the schema: preparation, warm-up, "main course", aftercare.