An acronym that breaks down into two parts:
The Alt stands for Alternative.
The Coin stands for Coin/Currency.
The umbrella term used to refer to all crypto assets that are not Bitcoin / Ethereum. (This includes Monero, Stellar, Cardano, and so on.)
Alt Coins generally position themselves to have higher technical capabilities than Bitcoin & Etherem. Nevertheless, there are extremist segments of the crypto industry that deny the validity of Altcoins.