OAuth

An open standard for authorisation that lets users grant apps access to their accounts on other services, without sharing passwords. Powers "Login with Google" and similar flows.

OAuth is behind every "Sign in with Google/Apple/Facebook" button. It's also how you connect apps: Google Calendar to your tool, Shopify to your dashboard, Stripe to your platform.

A founder's app uses OAuth to let users connect their Google Calendar. Users click "Connect Google Calendar," approve access, and their events appear, no password sharing.