I am 
so smart! I very quickly realized that the answer PRIMETIME referred to prime numbers and units of time.
Okay, okay, I have to be honest: I didn't even 
think about prime numbers. I thought PRIMETIME was a reference to the nightly 8-to-11 blocks of television programs and the four theme answers were simply phrases containing a word that designates time. During my school years, I liked television much more than I liked math. Can you tell? 

"Introduction" was PROEM, a word I was unfamiliar with. It means "an introductory discourse; preface; preamble" and comes from the Latin 
prooemium, adapted from the Greek 
prooímion, which means "prelude": 
pro + 
oím[ē] ("song") + the diminutive suffix 
ion.