Gateway

1. In a communications network, a network node equipped for interfacing with another network that uses different protocols.

Note 1: A gateway may contain devices such as protocol translators, impedance matching devices, rate converters, fault isolators, or signal translators as necessary to provide system interoperability. It also requires that mutually acceptable administrative procedures be established between the two networks.

Note 2: A protocol translation/mapping gateway interconnects networks with different network protocol technologies by performing the required protocol conversions.

2. Loosely, a computer configured to perform the tasks of a gateway.

Source: from Federal Standard 1037C and from MIL-STD-188

Also, Gateway refers to a computer company.