chiefly

definition: 
for the most part
sentence: 
Heroin abuse is soaring, thanks chiefly to its cheap price and widespread availability.
difficulty: 
very easy
toefl frequency: 
2
toefl classification: 
adverb: 
chiefly
adjective: 
chief
adjective sentence: 
It’s hard to say when the desire to be great became her chief ambition.
Corn is the chief corp of the Midwest.