What are a security token and a security token service (STS)?

A security token is a data package containing claims about a subject that can be used to authorise access to a resource; a security token service is a service that issues security tokens according to the WS-Security specification to authenticate subjects and give them access to protected resources.

The term "token" has a different meaning in cryptocurrency and blockchain contexts.

Go deeper:

• Security token service (Wikipedia) — issuing, validating, renewing and cancelling tokens (WS-Trust).
• JSON Web Token (Wikipedia) — the most common security-token format today.