/etc/shadow is a file on UNIX systems which contains the password information for the system's accounts and optional aging information.
Long ago it was considered safe to have encrypted passwords openly visible in the password file. When computers got faster and people got more security-conscious, this was no longer acceptable. Julianne Frances Haugh implemented the shadow password suite that keeps the encrypted passwords in the shadow password database (e.g., the local shadow password file
/etc/shadow, NIS, and LDAP), readable only by root.
Source: Linux Programmer's Manual
