PAM
PAM: Pluggable Authentication Modules
https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/7/html/system-level_authentication_guide/pluggable_authentication_modules
PAM is pluggable because there is a PAM module for different types of authentication sources (such as Kerberos,
sssd(8)
, NIS, or the local file system). Different authentication sources can be prioritized.