A progressive jackpot is a prize pool that grows with every bet. Part of each stake feeds the bank until someone hits the jackpot — then the pool resets to its starting value.
Networks and tiers
Jackpots can be local (one game) or networked (several games from one operator). Often there are tiers: from mini jackpots that hit often to grand ones that build for months.
Mega Moolah — the Microgaming network jackpot
Wolf Gold — three fixed jackpots
Age of the Gods — four Playtech tiers
RTP and the jackpot
For progressive games, RTP refers to the base game and excludes jackpot triggers. Part of the mathematical return is "frozen" in the growing pool.