Abbreviation for Transmission Control Protocol. TCP is a set of rules that defines how data is shared among computers. It is often used along with the Internet Protocol (IP) to send data in the form of message units between computers over the Internet. While IP takes care of handling the actual delivery of the data, TCP takes care of keeping track of the individual units of data (called packets) that a message is divided into for efficient routing through the Internet. Together these two protocols are known as TCP/IP (spoken “Tee – See – Pee over Eye Pee,” or just the letters TCPIP for short).
When data is sent to you from some server (say, a web server, for example) the Transmission Control Protocol (TCP) program in that server divides the file into one or more packets, numbers the packets, and then forwards them individually to the IP program. Although each packet has the same destination IP address, it may get routed differently through the network. At the other end (the client program in your computer), TCP reassembles the individual packets and waits until they have arrived to forward them to you as a single file.
TCP is known as a connection-oriented protocol, which means that a connection is first established, confirmed, and then maintained until such time as the message or messages to be exchanged by the application programs at each end have been exchanged. TCP is responsible for ensuring that a message is divided into the packets that IP manages and for reassembling the packets back into the complete message at the other end.