principally
adv.mostly or mainly. You use this to talk about the most important reason or part of something.
adv.for the most part; chiefly or mainly. Used to identify the primary factor or component in a situation.
“The book is principally about the history of jazz.”
“The new law is principally concerned with protecting the privacy of online shoppers.”
“The decline in the local population was principally attributed to the closure of the steel mill, which had been the town's primary employer for decades.”
Typically placed before the adjective or prepositional phrase it modifies, or between the auxiliary and main verb.
“What evidence suggests that school leavers motivated principally by reduced university fees would remain in demanding social care roles for a full year?”
“The evidence we have suggests Roman soldiers were paid principally in coins, especially silver coins.”
From Middle English principaly. By surface analysis, principal + -ly.
Etymology adapted from Wiktionary, available under CC BY-SA 4.0.