ICMP is the Internet Control Message Protocol which works at Layer 3 or the Network layer. It is used for all kinds of network messaging services utilized by IP, the Internet Protocol.
In fact, here’s how it is defined by Wikipedia:
“The Internet Control Message Protocol (ICMP) is one of the core protocols of the Internet Protocol Suite. It is chiefly used by the operating systems of networked computers to send error messages indicating, for example, that a requested service is not available or that a host or router could not be reached. ICMP can also be used to relay query messages”
(see RFC 1256, if you are into that kind of thing).
