An Acronym for Transfer Control Protocol / Internet Protocol. The name of the network protocol stack on which the entire Internet is built.

TCP/IP actually consists of a whole bunch of different protocols. At the lowest harware-independent layer (the network layer) is IP, which deals with getting packets of data from one machine to another and how to route them through other machines along the way, on a "best-effort" basis with no actual guarantee that they will make it to their destination. On top of this are built UDP and TCP, both of which are transport-layer protocols--they are how a program running on one machine can communicate with a program running on another machine. UDP, like the lower-level IP, makes no guarantees about reliable delivery or even that packets will arrive in the same order in which they were sent, whereas TCP is a full connection-oriented protocol which guarantees delivery and in the right order, or your money back.

There is another low-level protocol, ICMP, which is concerned with reporting issues with the state of the network. The ping(8) command makes use of one type of ICMP message.

Then there is a whole bunch of application-level protocols, built on top of TCP or UDP, which are what people use to do actual work: SMTP, HTTP, RTSP, SSH etc. DNS is also technically an application-level protocol, even though most users would view it as a part of the infrastructure of the Internet.