Expatiate
/ɪkˈspeɪʃieɪt/
Definition 1 of 2
Pro Tip 1/3Extended Discussion
Use 'expatiate' when you want to highlight that someone is going into great detail. 
The professor began to expatiate upon the implications of the study.
Pro Tip 2/3Not Always Positive
'Expatiate' can imply that the speaker is being overly detailed, potentially to the listener's annoyance. 
He's likely to expatiate on trivial matters, much to the audience's chagrin.
Pro Tip 3/3Formal Contexts
'Expatiate' is more likely used in formal speech or writing, not everyday conversation. 
In her thesis, she expatiated on the historical context of the law.