The process of verifying a user's or system's identity before granting access to data, an API, or a system.
Authentication answers the question "who are you?" It is distinct from authorization, which answers "what are you allowed to do?" Common authentication methods include passwords, API keys, OAuth tokens, and multi-factor authentication (MFA).
For APIs, authentication is typically implemented using API keys (a unique token included in each request) or OAuth (a protocol that allows users to grant applications access to their data without sharing passwords).
Example: A developer wants to access a government open data API that requires authentication. They register for an account and receive an API key — a long string of characters like sk_live_abc123xyz. They include this key in every API request as a header or query parameter. The API server verifies the key before processing the request. If the key is invalid or missing, the request is rejected.