An altcoin is any cryptocurrency other than Bitcoin. The term groups together the thousands of alternative coins and tokens that launched after Bitcoin, from large established networks to small experimental projects.
Altcoins vary widely in design: some change the consensus method, block speed, or supply schedule, while others add programmable features such as smart contracts. Many run on their own blockchain, while others are tokens issued on an existing chain like Ethereum.
Bitcoin is the reference point the term defines against: it was the first cryptocurrency, and "altcoin" simply means "alternative to Bitcoin". Within the altcoin group, stablecoins are a further subset designed to track a steady value rather than float freely.
You build a 1,000 USD crypto position split across two assets.
You put 600 USD in Bitcoin and 400 USD in altcoins:
400 USD / 4 altcoins = 100 USD per altcoin
The Bitcoin portion is not an altcoin; only the remaining 400 USD spread across the four alternative coins counts as your altcoin exposure.