1. an important part of something
Professional
Social
she was instrumental in my success
As a verb, 'to be instrumental' in something means to be an important or intrinsic part of it. You might have been 'instrumental in setting up' something or 'instrumental in researching' something, for example. This is a verb used more often in formal situations. Similar phrases include 'vital in/to' and 'to be a big part of'.