mesonium

The story

1. Proto-Indo-European*me
2. Proto-Indo-European*dʰeh₁-?
3. Proto-Indo-European*médʰi
4. Proto-Indo-European*médʰyos
5. Proto-Hellenic*métsos
6. Ancient Greekμέσος (mésos)
7. Englishmeso-
8. Ancient Greekἤλεκτρον (ḗlektron)
9. Latinēlectrum
10. New Latinēlectricus
11. Englishelectric
12. Proto-Indo-European*h₁ey-
13. Proto-Indo-European*h₁éyti
14. Proto-Hellenic*éimi
15. Ancient Greekἰόν (ión)
16. Englishion
17. Englishelectronbf.
18. Englishmeson
19. Englishonium
20. English todaymesonium

From meson + -onium.