... means the former students of a school or university. "Alumni" is the plural, and "alumnae" is the female plural. The singular for a male is "alumnus", whereas a female is "alumna".
... is the formal name for the graduate body and acts within the University as the voice for alumni. It also runs a series of events and makes an annual award to students. All past students and staff of the University are automatically members of Convocation.
You can find out more about Convocation in the Convocation pages of this site.