A graduation ceremony at a university or college.  See  http://www.deccanherald.com/content/319603/president-delhi-universitys-convocation.html.In India the word convocation is used almost exclusively to refer to a graduation ceremony. By comparison, many American universities welcome new students at a ceremony that they describe as a convocation, See  Stanford University example. Other gatherings at universities in America may also be called convocation, as in "gathering of people."