Biometric simply mean life measurement. Because bio means life while metric is measurement. So the bio deal with the use of human features or using life features in measurement to control a security system tagged security access control. So the aspect or part of human or life characteristics uused are fingers, full palm surface, feat surface, human walking, voice and retina.
All this mentioned or listed human parts actually determine the characteristics required and by extension the type of access control each of the system, human characteristics is been collected into the system through the module called biometric module and stored as access point requirement in the firmwares.
So, in my own understanding, we have finger print access control, palm access control, retina biometric access control, voice access control, etc