In the phonetic alphabet, Echo represents the letter E. The definition of echo - per my ever so reliable American Heritage Dictionary - is 1. Repetition of a sound by reflection of sound waves from a surface 2. The sound produced in this manner 3. Any repetition or imitation of something, as of the opinions, speech, or dress of another. Twins often echo one another, for example.
In Greek Mythology, Echo was a nymph whose unrequited love for Narcissus caused her to pine away until nothing but her voice remained.