1. a period of time (or a meeting) devoted to dealing with or carrying out a specific activity
a training/working/briefing session
A 'session' is a noun referring to a duration of time during which an official meeting takes place. A 'session' can refer to any time that is dedicated to a specific activity such as a 'training session' or a 'relaxation session'. An amount of time spent by a musician in a studio is referred to as a 'recording session'. Informally and colloquially, a 'session' can refer to a period of heavy drinking or partying.