JWT JSON Web Token
A signed text string that proves who you are — common for API authentication and single sign-on.
Three parts separated by dots: header, payload (containing e.g. user ID), signature. The server signs with a secret; the client sends the JWT with every call. The server verifies the signature and knows who's calling without hitting the database.
Often paired with OAuth. Not encrypted, only signed — don't put secrets in the payload, anyone can read the contents.