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 (30 December 2018):

firewall

   1. firewall code.

   2. firewall machine.