A session is a continuous period of play, from when you start to when you stop. Most slot results that people remember, good or bad, are session results, and they can sit a long way from a game's RTP.
Short sessions are dominated by volatility, so the outcome of one session says very little about a game. The cost of a session, on average, is total staked × (1 − RTP). Total staked grows with stake size and with time: at the GB minimum of 2.5 seconds per spin, an online slot cannot exceed 1,440 spins in an hour.
GB-licensed online slots must show how long you have been playing and your net position during play. Many people also use reality checks, deposit limits or a time-out to keep sessions within a plan.
Deciding before you start how long and how much a session will be, and stopping at that point whatever the result, is the approach recommended on our safer gambling page.