Collins COBUILD Advanced Learner's English Dictionary - mostly

Frequency: The word is one of the 3000 most common words in English. You use mostly to indicate that a statement is generally true, for example true about the majority of a group of things or people, true most of the time, or true in most respects. I am working with mostly highly motivated people... Cars are mostly metal. ...men and women, mostly in their 30s... = mainly ADV: ADV with cl/group