Maximum supply is the hard limit on how many units of a cryptocurrency can ever exist, written into the project's rules. Once that ceiling is reached, no new coins are created. Some coins enforce a fixed cap; others are designed with no maximum at all.
A capped supply is often presented as a defence against inflation: if no more can be made, existing holders cannot be diluted by endless new issuance. Bitcoin's 21-million cap is the best-known example of this scarcity-by-design.
A cap is not magic, though. Scarcity only supports value if there is genuine demand, and a fixed maximum says nothing about how many coins will unlock and when. Compare maximum supply with circulating supply to judge how much issuance is still to come.
Worked example
Bitcoin's rules cap the total that can ever exist at 21 million coins.
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This definition is general education, not investment advice. Markets — especially crypto — are volatile and you can lose money. Please read our disclaimer and see our methodology.