in large part; mainly or chiefly
"These accounts are largely inactive"
usually; as a rule
"by and large it doesn't rain much here"
Mainly or chiefly; for the most part; usually, generally, on the whole.
"The new policy applies to all employees mostly, though there are a few exceptions for temporary staff."
In plain English: Mostly means that something happens most of the time but not always.
"Mostly everyone enjoyed the party last night."
Usage: Use mostly to indicate that something happens in the majority of cases but not always, distinguishing it from words like mainly which imply a stronger sense of exclusivity. Place mostly before the verb or adjective it modifies to show general prevalence without absolute certainty.
Mostly is formed by adding the suffix -ly to most. This construction simply means "for the greater part."