On the XMPP/Jabber network users will see ping sent every now and then. A user will typically send out a ping to check to see if they're still connected to their Jabber service, the amount of lag to a conferencing service, and for some as a check to see if anyone is actually home.

The history of using ping for such purposes goes all the way back to the use of sonar. Sonar systems would emit a ping and wait for the pong creating the typical piiing pooong heard in movies about submarines. This terminology was later adopted by the creator of the ping utility used to test reachability, lag, and packet loss on the Internet. The use of ping on the Jabber network most likely comes from that utility. It's use is essentially for the same purpose, minus information such as an accurate measurement of lag.

Due to the above history, the appropriate response for a ping is a pong, just like sonar. So whenever you see a ping in a conference, you now know what to say: POOOONG!