Altseason, short for altcoin season, describes a period when many cryptocurrencies other than Bitcoin outperform Bitcoin. The emphasis is on broad relative performance, rather than one token’s isolated rally. It is market slang, not a scheduled calendar season or a guarantee that every altcoin will rise.
How people measure it
Different observers use different asset baskets and time windows. CoinMarketCap’s Altcoin Season Index compares a selected top-100 coin universe against Bitcoin over a rolling 90 days, excluding stablecoins and wrapped tokens. Its published rule labels the period altcoin season when 75% outperform Bitcoin. Ethereum is included in that provider’s altcoin universe.
That threshold belongs to a particular methodology; it is not a universal definition. Falling Bitcoin dominance can accompany altcoin strength, but dominance measures market-cap shares rather than the number of outperforming coins.
Relative strength has limits
Outperforming Bitcoin does not necessarily mean making money in a fiat currency: an altcoin falling less than Bitcoin can still outperform it. Performance also varies sharply between tokens and sectors. An index summarizes an observed window, not the next one, and broad enthusiasm can reverse before a late participant realizes a gain.