TOTP (Time-based One-Time Password)
A one-time code, valid for about 30 seconds, generated from a shared secret and the current time — the algorithm behind most authenticator apps.
aka: TOTP, HOTP
TOTP is RFC 6238’s successor to counter-based HOTP: both device and server derive a code from a shared secret, but TOTP uses the current time instead of an incrementing counter, so codes rotate automatically without the two sides drifting out of sync from missed attempts. Clock drift beyond a small tolerance window is the main operational failure mode.