View image A computer application used in a network to prevent external users gaining unauthorised access to a computer system. A firewall may be software running in the main computer or a separate computer or other device physically located between the system and the external access. It limits the data and instructions that can be received or sent to external users. For example, the firewall could block certain types of data, only allowing access from specific computers or require additional user identification. It is usually used with a proxy server.