In audio signal processing and acoustics, an echo (plural echoes) is a reflection of sound, arriving at the listener some time after the direct sound. Typical examples are the echo produced by the bottom of a well, by a building, or by the walls of an enclosed room and an empty room. A true echo is a single reflection of the sound source. The time delay is the extra distance divided by the speed of sound. The word echo derives from the Greek ἠχώ (ēchō), itself from ἦχος (ēchos), "sound".

If so many reflections arrive at a listener that they are unable to distinguish between them, the proper term is reverberation. An echo can be explained as a wave that has been reflected by a discontinuity in the propagation medium, and returns with sufficient magnitude and delay to be perceived. Echoes are reflected off walls or hard surfaces like mountains and privacy fences.

When dealing with audible frequencies, the human ear cannot distinguish an echo from the original sound if the delay is less than 1/10 of a second. Thus, since the velocity of sound is approximately 343 m/s at a normal room temperature of about 25°C, the reflecting object must be more than 17.15 m from the sound source at this temperature for an echo to be heard by a person at the source.

Echo most commonly refers to Echo (phenomenon), the reflection of a sound.

It may also refer to:

An Echo is a multiple reflection of sound which can happen in a covered area

A phenomenon (from Greek φαινόμενoν), plural phenomena, is any observable occurrence. Phenomena are often, but not always, understood as 'appearances' or 'experiences'. These are themselves sometimes understood as involving qualia.

The term came into its modern philosophical usage through Immanuel Kant, who contrasted it with noumenon (for which he used the term Ding an sich, or "thing-in-itself"), which, in contrast to phenomena, are not directly accessible to observation. Kant was heavily influenced by Leibniz in this part of his philosophy, in which phenomenon and noumenon serve as interrelated technical terms.

In scientific usage, a phenomenon is any event that is observable, however common it might be, even if it requires the use of instrumentation to observe, record, or compile data concerning it. For example, in physics, a phenomenon may be a feature of matter, energy, or spacetime, such as Isaac Newton's observations of the moon's orbit and of gravity, or Galileo Galilei's observations of the motion of a pendulum.

Mesut Özil (German pronunciation: [ˈmeːzut ˈøːzil], Turkish pronunciation: [me̞ˈsut ˈø̞zil]) (born 15 October 1988) is a German footballer who plays for Spanish La Liga club Real Madrid and the Germany national football team. Özil has been a youth national team member since 2006, and a member of the German national team since 2009. He gained international attention during the 2010 FIFA World Cup and was nominated for the Golden Ball Award, which is awarded to the tournament's best player. Following his breakout performance at the World Cup, he was transferred from Werder Bremen to Real Madrid in August 2010.

Özil is acclaimed for his finesse and improvisation as an attacking midfielder. His style and knack for providing assists for his team-mates has been compared to that of Real Madrid legend Zinedine Zidane. In 2011, he ranked second in assists in La Liga with 17, and first in major European competitions with 25.

Özil began his youth career playing for various clubs in Gelsenkirchen and then played five years for Rot-Weiss Essen.