인증이 · 인증을
certification, authentication, verification
Example
로그인하려면 휴대폰 인증을 해야 해요.
To log in, you have to verify with your phone.
Word parts
인 recognize+증 proof
Literally 'recognize and prove'.
Good to know
Slang 인증샷 means a photo posted as proof that you did something.