écho

Écho is the phenomenon of a sound being reflected back to the listener after bouncing off a surface, creating a perceptible repetition of the original sound. In acoustics, an echo is distinguished from reverberation: an echo is a distinct, time-delayed copy of the initial sound, whereas reverberation is the persistence of sound caused by many overlapping reflections.

The word derives from the Greek mythological nymph Echo, and ultimately from the Greek ēkhō meaning "I sound" or "sound." In French, écho is used for the physical phenomenon and, in literature and mythology, as a reference to the mythic Echo, who could only repeat the words of others.

In science and medicine, echoes appear in various technologies. Echolocation is the biological use of emitted sounds and their echoes to determine distance or shape of objects, seen in bats and dolphins. In medical imaging, ultrasound relies on echoes produced by high-frequency sound waves to visualize internal tissues; informally, a medical ultrasound exam is often called une écho in French.

In culture, the figure of Echo has inspired poetry and music, symbolizing repetition or memory. The term also appears in brand names, literature, and fictional works, sometimes as a reference to resonance or reflection.