An Etymological Dictionary of Astronomy and Astrophysics

One of two or more atoms having the same number of protons in its nucleus, but a different number of neutrons and, therefore, a different mass.

Etymology (EN): Isotope, from → iso- + -tope, from Gk. topos “place.”

Etymology (PE): Izotop, loan from Fr., as above. hamjâ, from ham- “together” → com- + jâ “place” (from Mid.Pers. giyag “place;” O.Pers. ā-vahana- “place, village;” Av. vah- “to dwell, stay,” vanhaiti “he dwells, stays;” Skt. vásati “he dwells;” Gk. aesa (nukta) “to pass (the night);”
Ossetic wat “room; bed; place;” Tokharian B wäs- “to stay, wait;” PIE base ues- “to stay, live, spend the night”).