Echo equates to return loss in your cable plant. Echo can be used for diagnostic purposes, but for the most part, echo is cancelled via echo cancellers and through replacement of faulty cabling channels. When a transmitting signal is returned on a receiving signal (or in simple terms, if you can think of it as a signal that bleeds over in the opposite direction) you have echo. If you make a regular VoIP call, for instance, there is some reflected noise. If you are close enough to the echo, you hear it before you can perceive it or it becomes audible. If there is a large distance between the caller and the sender or if the return loss is too great, the echo becomes audible and a big pain! If you have echo only calling outside your building, have your carrier check your line. If you have echo internally to your building, get someone to check your cabling for return loss. A certified infrastructure auditor can help.
In the days of half-duplex operations, local echo allowed key strokes to be echoed onto a screen as they were typed. This is an old term and I don't know if that is what you are referring to or not. Feel free to write back if you need more help.
This was first published in March 2005