Firewall
A firewall can be software or hardware designed to prevent
unauthorized electronic access to a networked computer system.
You can think of a firewall as a shell around your computer protecting
it from incoming attacks, blocking unauthorized Internet users from
connecting to your Internet network and personal computer.
There are also bidirectional firewalls that prevent, not just incoming
attacks, but also outgoing connections made from your
computer by malware trying to send spam or to connect to a remote server.