Altcoin
Any digital asset other than bitcoin; a loose market label rather than a technical category.
The word is a contraction of alternative coin, dating from the period when almost every new project was a modified copy of Bitcoin's code. Today it is a residual bucket that holds everything from independent smart-contract platforms to memecoins and tokens issued on other chains, and it describes nothing about a design. Definitions vary between data sources: some treat everything except bitcoin as an altcoin, others exclude ether as well, and some exclude stablecoins. Because the bucket is defined by what it is not, any statistic about altcoins depends entirely on the coverage rules of whoever produced it.
In practice
A chart of altcoin market share is really a chart of the total market capitalization the provider tracks minus bitcoin, so adding newly listed assets changes the line without any price moving.
The common misunderstanding
Altcoin is not a technical classification: assets in the bucket share no common architecture, issuance policy, or purpose beyond not being bitcoin.