A utility available on all operating systems that helps test network connectivity.
Ping sends echo request packets to the specified IP address and waits for a response. If there's no response then the address is deemed unreachable at that time.
A trace route utility - "tracert" on Windows for example - can be used to help find out where the break in connection is.
As stated on Wikipedia, ping was named:
after the sound pulses of active sonar, which also involves sending a signal and measuring the time until any echo is received