Default gateway

A gateway is a node (a router) on a computer network that serves as an access point to another network.
A Default Gateway (Def.GW) is the node on the computer network that is chosen when the IP address does not belong to any other entities in the Routing Table.
In homes, the gateway is usually the ISP-provided device that connects the user to the Internet, such as a DSL or cable modem.
In enterprises, however, the gateway is the node that routes the traffic from a workstation to another network segment. The default gateway is commonly used to be the node connecting the internal networks and the outside network (Internet). In such a situation, the gateway node could act as a proxy server and a firewall. The gateway is also associated with both a router, which uses headers and forwarding tables to determine where packets are sent, and a switch, which provides the actual path for the packet in and out of the gateway.
In other words, it is an entry point and an exit point in a network.
Usage
A default gateway is used by a host when an IP packet's destination address belongs to someplace outside the local subnet (thus requiring more than one hop of Ethernet communication). The default gateway address is usually an interface belonging to the LAN's border router.