An Etymological Dictionary of Astronomy and Astrophysics

General: Any of a set of physical properties whose values determine the characteristics or behavior of something. → impact parameter; → ionization parameter.
Math.: A constant or variable term in a function that determines the specific form of the function but not its general nature, as a in f(x) = kx, where k determines only the slope of the line described by f(x).

Etymology (EN): Mod.L. parametrum, from Gk. → para- + metron “measure,” → meter.

Etymology (PE): Parâmun, from parâ-, → para-,

• mun/mân “measure,” as in Pers. terms pirâmun “perimeter,” âzmun “test, trial,”
  peymân “measuring, agreement,” peymâné “a measure; a cup, bowl,”
  from O.Pers./Av. mā(y)- “to measure;” cf. Skt. mati “measures,” matra- “measure;” Gk. metron “measure;” L. metrum; PIE base *me- “to measure.”