A biometric liveness check refers to the verification that the features being presented to the biometric application are those of a living subject, and not a copy or imitation of those features.
A liveness check allows a biometric application to discriminate in between the real biometric factor of a subject and artificial copies of those features making up the biometric factor. Liveness detection reduces the likelihood of spoofing attempts to succeed, and as such reduces the false acceptance rate.
An example of a liveness check is a face recognition software requiring the subject to blink with his eyes, as such the software avoids spoofing through the use of photographs.