Here’re two online introductory courses for entomology:
- https://projects.ncsu.edu/cals/course/ent425/library/tutorials/index.html
-
https://www.amentsoc.org/insects/
This one has a glossary section:
https://www.amentsoc.org/insects/glossary/
And this classic entomological dictionary is freely available:
https://www.biodiversitylibrary.org/bibliography/10098#/summary
(Jardine’s Dictionary of entomology)