One-time password (OTP)
Also called: OTP, Verification code
A short code that works for one login and a short time, sent by SMS or email or generated by an app.
Definition
OTPs are the most common second factor. Testers check expiry, reuse, resend, wrong attempts and how the code is entered (autofill, paste, leading zeros).
In practice
A 6-digit code 042917 arrives by SMS; it should expire after 5 minutes and stop working once used.