Source : Webster's Revised Unabridged Dictionary (1913)

Parfocal \Par*fo"cal\, a. [Pari- + focal.] (Opt.)
   With the lower focal points all in the same plane; -- said of
   sets of eyepieces so mounted that they may be interchanged
   without varying the focus of the instrument (as a microscope
   or telescope) with which they are used.