NOUN (3)

1. (colloquial) the application of maximum thrust;
- Example: "he moved the throttle to the firewall"

2. (computing) a security system consisting of a combination of hardware and software that limits the exposure of a computer or computer network to attack from crackers; commonly used on local area networks that are connected to the internet;

3. a fireproof (or fire-resistant) wall designed to prevent the spread of fire through a building or a vehicle;

WordNet (r) 3.0 (2006):

firewall
    n 1: (colloquial) the application of maximum thrust; "he moved
         the throttle to the firewall"
    2: (computing) a security system consisting of a combination of
       hardware and software that limits the exposure of a computer
       or computer network to attack from crackers; commonly used on
       local area networks that are connected to the internet
    3: a fireproof (or fire-resistant) wall designed to prevent the
       spread of fire through a building or a vehicle

The Free On-line Dictionary of Computing (19 January 2023):

firewall

    A dedicated gateway server with
   special security precautions on it, used to service external
   connections (typically from the public Internet) and to
   protect servers and networks hidden behind it from crackers.

   As well as filtering incoming traffic, a firewall should also
   filter outgoing traffic ("egress filtering") to avoid the
   embarrassment or data leaks that could be caused if the
   machine is compromised.

   A firewall may be a separate hardware unit, possibly a
   dedicated network appliance, or it may be implemented entirely
   in software, possibly running on a virtual machine.

   The typical hardware firewall is an inexpensive
   microprocessor-based Unix machine with no critical data,
   with public network ports on it, but just one carefully
   watched connection back to the rest of the cluster.  The
   special precautions may include threat monitoring,
   call-back, and even a complete iron box keyable to
   particular incoming IDs or activity patterns.

   The type of network and security environment of a firewall machine
   is often called a De-Militarised Zone (DMZ).  It may contain
   other servers such as e-mail servers or proxy gateways -
   machines that need to be publicly accessible but also need some
   access to internal systems.

   Also known as a (Venus) flytrap after the insect-eating plant.

   (2014-07-15)