An Etymological Dictionary of Astronomy and Astrophysics

1a) Moving along in a → stream; going as in a stream.

1b) A → dynamical system that evolves continuously with time.

1c) The transference of energy.

2. The corresponding verb.

Etymology (EN): O.E. flowan, from P.Gmc. *flo- (cf. Du. vloeien “to flow,” O.H.G. flouwen “to rinse, wash”), probably from PIE *pleu- “to flow, float” (cf. Skt. plavate “navigates, swims,” plavayati “overflows;” Gk. plyno “I wash,” pleo “swim, go by sea;” L. pluere “to rain;” Arm. helum “I pour;”
Lith. pilu “to pour out”).

Etymology (PE): 1) Tacân, from tac- variant tâz- present stem of tacidan, tâxtan, tâzidan “to run; to hasten; to assault,”

• noun and adj. suffix -ân. Related to the first component are
  Mod.Pers. tajan name of a river (initially “flowing, streaming, stream”), tâzi “swift (greyhound),” tak “running, rush,” from
  Mid.Pers. tâz-, tâxtan “to flow, to cause to walk,” tc- “to flow, to walk,” tag “running, attack,” tâzig “swift, fast;”
  Khotanese ttajs- “to flow, to walk;” Av. tac- “to run, to flow,” taciāp- “flowing water,” tacinti (3pl.pers.act.) “to flow,”
  tacar- “course,” tacan “current, streaming;” cf. Skt. tak- “to rush, to hurry,” takti “runs;” O.Ir. tech- “to flow;” Lith. teketi “to walk, to flow;” O.C.S. tešti “to walk, to hurry;” Tokharian B cake “river;” PIE base *tek^w- “to run; to flow.”
  

2. Verbal form.