In One-way Transport Layer Security (TLS), client who initiates the connection verifies the identity of the server or entity it is connecting to, but server not necessarily validates client identity.
In Two-way TLS connections, both client and server validates each other's certificate (public key) to achieve mutual authentication. The question here is, does the client necessarily send its certificate? What if the client sends a credentials (E.g token/API key) that the server knows and validates to verify the identity, is this not a mutual authentication and two-way TLS? I do understand certificates are a better way of proving identity than token/secret.