Glossary
ICO
Initial coin offering: a public token crowdsale used to fund a project's development. Mostly replaced by airdrops, IDOs, and IEOs after the 2018 regulatory pullback.
ICOs peaked in 2017 and 2018: a project would post a whitepaper, deploy a sale contract, and accept ETH or BTC in exchange for newly minted tokens. Some were excellent (Ethereum itself raised this way); many more were unregistered securities offerings or outright scams.
After the SEC and other regulators began enforcement against unregistered offerings, the mechanic gave way to IEOs (exchange- hosted sales), IDOs (DEX-hosted), and airdrops. "ICO" today is mostly a historical term.