A coin is the native cryptoasset of its own blockchain — it's used to pay transaction fees and secure the network that it runs on. Bitcoin (on the Bitcoin network) and Ether (on Ethereum) are both coins in this sense.
This is the usual distinction drawn between a "coin" and a "token": a coin has its own underlying blockchain, while a token is built on top of someone else's. In everyday conversation people often use the two words interchangeably, but the distinction is useful once you're reading technical material.