The Applause reference article from the English Wikipedia on 24-Jul-2004
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Applause

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Applause is the act of clapping, or striking the palms of the hands together, in order to create noise. Audiences are usually expected to applaud after a performance, such as a musical concert, speech, or play.

In most western countries, audience members clap their hands at random to produce a constant noise. As a form of mass nonverbal communication, it is a simple indicator of the average relative opinion of the entire group; the louder the noise, the stronger the sign of approval.

In some European countries, audiences clap rhythmically in unison, about two claps per second, to indicate approval. In some cultures, however, clapping in unison is understood as disapproval, and is an insult to the performer.

Famous performers, as well as celebrities in other fields such as politics, may also receive applause when they first appear on stage, before they deliver a speech or their first speaking part. This accolade given in response to his or her past achievements and is not a reflection of the performance the audience is attending.

On some occasions, applause can also occur in the middle of an event. The President of the United States, in his State of the Union address, is often interrupted by applause; in fact, tracking the number of such interruptions has become a trend for various television news channels.