OTP (one-time password)

A short code valid for a single login or verification, typically 4-8 digits delivered by SMS, voice call, or generated by an app.

An OTP is a code that works once and then dies. In SMS verification the service generates the code, texts it to the number you gave, and compares what you type against what it sent. Validity windows are short, usually one to ten minutes.

OTPs delivered by SMS are convenient because every phone can receive them; OTPs generated locally by an authenticator app (TOTP) are stronger because nothing travels over the phone network.