Final answer:
A PIN (Personal Identification Number) is a secret numeric password used for authentication, typically to gain access to systems such as ATMs or online accounts.
Step-by-step explanation:
The secret numeric password used for authentication is a PIN (Personal Identification Number). A PIN is typically a four to six-digit number that is used to verify the identity of the person trying to gain access to a system, such as an ATM, mobile phone, or online banking platform. It's one of the simplest forms of security measures that are based on the knowledge only the user is supposed to possess.
Other options listed, such as security tokens, digital certificates, and digital signatures, serve different roles in the domain of security and authentication. A security token is a physical or digital device that generates a secure and dynamic code, a digital certificate is a tool for verifying the ownership of a public key in encryption, and a digital signature is a way to ensure the integrity and authenticity of a digital document or message.