SecurID, now known as RSA SecurID, is a mechanism developed by Security Dynamics (later RSA Security and now RSA, The Security Division of EMC) for performing two-factor authentication

Two-factor authentication

Two-factor authentication is an approach to authentication which requires the presentation of two different kinds of evidence that someone is who they say they are. It is a part of the broader family of multi-factor authentication, which is a defense in depth approach to security...

 for a user to a network resource.
The RSA SecurID authentication mechanism consists of a "token

Security token

A security token may be a physical device that an authorized user of computer services is given to ease authentication...

" — either hardware (e.g. a USB dongle) or software (a soft token

Software token

A software token is a type of two-factor authentication security device that may be used to authorize the use of computer services. Software tokens are stored on a general-purpose electronic device such as a desktop computer, laptop, PDA, or mobile phone...

) — which is assigned to a computer user and which generates an authentication code at fixed intervals (usually 60 seconds) using a built-in clock and the card's factory-encoded random key

Key (cryptography)

In cryptography, a key is a piece of information that determines the functional output of a cryptographic algorithm or cipher. Without a key, the algorithm would produce no useful result. In encryption, a key specifies the particular transformation of plaintext into ciphertext, or vice versa...

 (known as the "seed".