The point of interconnection between a computer and a data network. It can be network interface card (NIC) or implemented in software.
A physical interface would commonly be to ethernet. The operating system generally maintains a table of virtual network interfaces in memory. In Linux, the virtual interfaces are to local (loopback) in and one or more named interfaces to named network domains. Without a network interface, packets will not leave the computer. One of the first tasks the network stack in Linux does is determine based on IP address, through which network interface to send the packet.