OTP
acronymsecuritycryptography
Stands for: one-time password
A password valid for a single login or a short window.
A one-time password is used once and then invalid, defeating replay of a captured code. TOTP (time-based) and HOTP (counter-based) are the two common ways to generate them for two-factor authentication.
One-time password covers both the codes an authenticator app generates and, confusingly, the one-time pad of classical cryptography. In practice the acronym almost always means the former, and the distinction is worth making because the security properties are nothing alike.
The authentication kind is a shared secret producing a short-lived code, strong against password reuse and credential stuffing and weak against real-time phishing, because a user can be induced to type the code into an attacker's page. The one-time pad is information-theoretically unbreakable and impractical, because the key must be truly random, as long as the message, used once and distributed securely, which is the same problem as sending the message.
Also known as: otp