photoreactivation

The story

1. Proto-Indo-European*bʰeh₂-
2. Proto-Indo-European*bʰéh₂os
3. Proto-Hellenic*pʰáwos
4. Ancient Greekφᾰ́ος (phắos)
5. Ancient Greekφῶς (phôs)
6. Ancient Greekφωτο- (phōto-)
7. Englishphoto-
8. Proto-Italic*wre-
9. Latinre-
10. Old Frenchre-
11. Middle Englishre-
12. Englishre-
13. Proto-Indo-European*h₂eǵ-
14. Proto-Indo-European*h₂éǵeti
15. Proto-Italic*agō
16. Latinagō
17. Ancient Greekἐνεργητικός (energētikós)
18. Latināctīvu
19. Old Frenchactif
20. Middle Englishactyf
21. Englishactive
22. Englishactivate
23. Englishactivation
24. Englishreactivation
25. English todayphotoreactivation

From photo- + reactivation.