You misunderstood my post.
@wrbtrader mentioned "explanation" when the discussion was about "definitions."
My point to them was: of course,
explanations can come later in a highly technical book.
No one will know what "H2" means inherently. What first comes to (my) mind is dihydrogen.
It should be defined at or by the time it is first encountered in a work; otherwise, that term just dangles in a fully functional mind while the reader continues reading the work.
Just to be clear...because things get so clusterfcked in online forums:
My points:
1. Stating that a proximate definition should be provided is not the same as stating that a proximate explanation should be provided.
2. No one inherently knows wtf "H2" means.
EDIT: To address your specific point. Let's say H2 truly needs no explanation. I won't know that until it is first defined. Once one reads the definition, it then becomes clear, and no explanation is desired/needed.
Or maybe a reader can 'figur' it out from the context. Again, that shouldn't be required.