Transmission Control Protocol (TCP)

Source

To overcome the unreliability of connectionless Internet Protocol (IP), Transmission Control Protocol (TCP) provides connection-oriented protocol that enable you to

• send longer message
• make sure your messages arrive at the destination

TCP is built on top of the basic IP protocol and adds sequence numbers, acknowledgements, and retransmissions.

Source

TCP is one of the protocol in the transportation layer ( See OSI 7 Layer Model); UDP (User Datagram Protocol) is another one.