Firewall
A filter on the way in and out, driven by rules. Default allow is a very expensive switch.
A firewall is a network device (or a host agent) that permits or blocks traffic according to a rule set. Direction, port, identity, and application are the usual knobs.
Rules that nobody reviews become archaeology. Any-any at the bottom is a confession. Pair it with segmentation so a hole on the web tier is not a hole on payroll.
A host firewall is still a firewall. Turning it off because an app vendor whined is a change. Ticket it.
Fact source: ASD's ACSC glossary.