The process of verifying the identity of a user, system, or application before granting access to resources. Like showing your ID at a security checkpoint to prove you are who you claim to be.
When you log into your email with a username and password, the system performs authentication to verify your identity before showing your inbox.
All four provide identity services used to authenticate users and workloads. AWS IAM and OCI IAM manage identities and authentication for their clouds; Microsoft Entra ID and Google Cloud Identity provide directory-based authentication and SSO, and can integrate with cloud resource access.