An Etymological Dictionary of Astronomy and Astrophysics

1. An ordered recurrent alternation of strong and weak elements in the flow of sound and silence in speech; a particular example or form of rhythm.
   

2. The aspect of music comprising all the elements (as accent, meter, and tempo) that relate to forward movement.
   

3. A regularly recurrent quantitative change in a variable biological process (Merriam-Webster.com).

Etymology (EN): From L. rhythmus “movement in time,” from Gk. rhythmos “measured flow or movement, rhythm; proportion, symmetry; arrangement,” related to rhein “to flow,” from PIE root *sreu- “to flow”

Etymology (PE): Ritm, loan from Fr.