Token is a kind of any existing debt holding, or services contract; a person, who has launched a token, is obliged to provide a buyer with anything useful, service, or good. In a narrow sense, token is a digital asset initiated by a private individual, or legal body with a specific purpose (to be used as stocks in course of ICO fundraising for instance).
Sometimes, tokens are called cryptocurrencies and added to altcoin number. However, it is not quite a commonly accepted token denomination due to there are significant differences between tokens and ordinary cryptocurrencies. It would be better to consider tokens as a secondary structure of the initial cryptocurrency, and its blockchain.