Rhythm is defined by Noah Webster's 1828 dictionary as the variety in the movement as to quickness or slowness, or length and shortness of the notes; or rather the proportion which the parts of the motion have to each other.
We use rhythm continually in our daily lives through speech, writing, music and other forms of entertainment. Most commonly rhythm is associated with music. In western music rhythm is maintained in a time signature that is usually universally accepted. When learning a new instrument students can set an electronic or manual metronome to the rhythm of the time signature to keep time as they play. In respect to music, rhythm shows up in multiple ways within a song including:
In addition to time signature, rhythm is also measured using a term called tempo in Western music. Tempo is usually measured in 'beats per minute where 60bpm means a speed of one beat per second. A rhythmic unit is a durational pattern which occupies a period of time equivalent to a pulse or pulses on an underlying metric level, as opposed to a rhythmic gesture, which does not. Together time signature, tempo and rhythmic unit help us to measure rhythm in various types of music.
Some genres of music make different use of rhythm than others, for example most Western music is based on divisive rhythm while non-Western music uses more additive rhythm. African music makes heavy use of polyrhythm. Indian music uses complex cycles and Balinese often uses complex interlocking rhythms. A lot of Western classical music is fairly rhythmically simple staying in a simple meter. On the contrary, the widespread use of irrational rhythms in New Complexity began to surface in the 20th century. Composers like Igor Stravinsky, Philip Glass, Steve Reich and modernists like Olivier Messiaen used increased complexity. They used odd meters and techniques such as phasing and additive rhythm.
The vast understanding of rhythm comes from a diligent study known as prosody. This process consists of a focused study of rhythm, stress and pitch in speech. There are three categories of prosodic rules which create rhythmic successions. These categories are additive; open-ended and repetitive (same repeated duration), cumulative; closure or relaxation (short-long), or counter cumulative; openness or tension (long-short).