fluctuation

Short definition from Wiktionary

Noun

1. 1
   A motion like that of waves; a moving in this and that direction; an irregular rising and falling.
2. 2
   A wavering; unsteadiness.
3. 3
   In medicine, a wave-like motion or undulation of a fluid in a natural or abnormal cavity (e.g. pus in an abscess), which is felt during palpation or percussion.