Echo or reverberation
The terms echo and reverb (reverberation) are often used on location interchangeably. Being a sound recordist you have to explain sonic phenomena to people in a quick and simple way. I am always trying to educate everyone to save any future confusion. So should you say echo or reverberation?
Echo is essentially one reflection of a complex sound wave (speech is one example) that comes straight back to you. Imagine the sound waves (complex sound wave) is a tennis ball. You hit the tennis ball perpendicular to a big concrete wall (90°) the ball will come straight back to you, this is an echo.
You try a second time, however this time you hit the ball at a 45° angle to the wall and it bounces off at 135° (45° + 90°) because of the angle it came in at. The wall would need a few more surfaces to bounce off before it comes back to you. So if you again hit it at 45° in a room it would bounce of several walls. This is reverberation.
This is why you can add reverb later in post production.
There will only be a few instances when you will be talking about an echo when you are on location, generally in closed spaces you talk about reverberation (reverb)