echo

Echo is a term with multiple meanings across science, mythology, and technology. In acoustics, an echo is the perception of a sound reflected from a surface and returning to the listener. For a distinct echo rather than reverberation, the sound must be heard after a delay typically on the order of about 0.1 seconds or more, with distance, atmosphere, and surface properties shaping the effect.

In Greek mythology, Echo was a mountain nymph cursed by Hera to repeat the last words spoken to her. The tale links Echo to themes of speech and memory, including her involvement with Narcissus, after which she faded to a voice that could only repeat others’ words.

Echo also refers to a pair of early space-science experiments. Echo 1 (1960) and Echo 2 (1964) were large reflective balloons used as passive communications satellites. They worked by reflecting radio signals transmitted from Earth back to other locations, demonstrating long-distance radio communication without onboard power sources.

In computing, the command echo outputs its input to the standard output. It is a fundamental utility in many operating systems and scripting environments, commonly used to display messages or the values of variables during program execution.

Echo is also associated with echocardiography, a medical imaging technique that uses ultrasound to visualize the heart’s structure and assess its function. It is a common, noninvasive diagnostic tool in cardiology and critical care.

Finally, Amazon Echo is a family of smart speakers and related devices that integrate the Alexa voice assistant. These devices enable hands-free control of music, information retrieval, smart-home automation, and third-party integrations.