Stands for "Internet Protocol." IP provides a standard set of rules for sending and receiving data over the Internet. It allows devices running on different platforms to communicate with each other as long as they are connected to the Internet.
In order for a Internet-connected host to be recognized by other devices, it must have an IP address. This may be either an IPv4 or IPv6 address, but either way it uniquely defines a device on the Internet.
The Internet Protocol also provides basic instructions for transferring packets between devices. However, it does not actually establish the connection or define the ordering of the packets transmitted. These aspects are handled by the Transmission Control Protocol, which works in conjunction with the Internet Protocol to transfer data between systems on the Internet. For this reason, connections between Internet-connected systems are often called "TCP/IP" connections.
NOTE: IP may also be short for "IP address," as in "What is your IP?" In this case, IP refers to the unique identifier of a system, not the protocol itself.