Telemetry is gameplay data captured during play: deaths, retries, completion times, route choices, skill use, purchases, retention, input accuracy, and other measurable traces of behaviour. It is the raw material for game-analytics, player-modelling, balancing, and adaptive systems. Yannakakis and Togelius treat telemetry as one of the key inputs for modelling players, because it captures what the player actually does rather than what they say they do. (Yannakakis and Togelius, Artificial Intelligence and Games, see source-ai-and-games; CRE342 Lectures, see source-cre342-lectures)
Telemetry is descriptive before it is interpretive. The design work begins when a team decides which behavioural traces matter and what they can legitimately infer from them.
Related: game-analytics, player-modelling, machine-learning-games, overview-ethical-game-design