Progression Stages
Within the Cookie Clicker community, a set of progression stages have been defined, based on a set amount of requirements, to quickly determine where one's progress lies within the game and what tips/strategies can be given to work.
These stages are not defined in the game itself and only serve when communicating with the rest of the community.
Early-game
Starts: immediately on a new save
Ends: after first 'optimal' ascension (at 365 prestige levels)
Early-game begins with a slow pace to get the player acquainted with the game, revolving around a steady accumulation of cookies through buildings and upgrades.
After obtaining a sugar lump to level up the wizard tower and unlocking the Grimoire, simple combos utilizing only 2 golden cookies may be executed. Once all golden cookie upgrades have been bought, combos utilizing 3 golden cookies may be executed.
If one ascended with significantly less than 365 prestige levels, it will still be considered early game. However, ascensions with an amount close to 365 is debatable to have entered midgame.
Mid-game
Starts: after first 'optimal' ascension (at 365 prestige levels)
Ends: after acquiring all heavenly upgrades
This is usually the shortest stage of the game, as prestige levels and heavenly upgrades begin manifesting absurd multipliers.
The Wall
Around septendecillion to novemdecillion cookies, the game starts to slow again and simple combos (such as Frenzy + Click Frenzy) don't seem to move the needle anymore. Utilizing Krumblor's auras, Pantheon gods, and dualcasting in the Grimoire will be required to get past it.
Late-game
Starts: after acquiring all heavenly upgrades
Ends: after acquiring enough cookies (1 trevigintillion in one ascension) to obtain every cookie-related achievement
More advanced combos are required to get to endgame. This includes things like Garden setups, quadcasting, and loans.
Endgame
Starts: after acquiring enough cookies to obtain the last cookies baked this ascension achievement
This stage never ends.