Online security device is a term that refers to a firewall, which is an 8-letter solution to the crossword. A firewall is a network security system that monitors and controls incoming and outgoing network traffic based on predetermined security rules. It is designed to protect private networks from malicious activity, such as unauthorized access, viruses, and other cyber threats. Firewalls can be either hardware or software-based, and are used to protect both public and private networks.