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Some different answers on this.
One standard used by US News is the number of graduates that can be solicited and this tends to be the most common.
Many schools also define an alum as anyone who attended a certain number of semesters or years. This is often done so that individuals that may have attended but not necessarily graduated are included in solicitation pools and as part of the overall marketing strategy. In terms of giving and connection with an institution, it is often the case that those who attended often have just as many strong ties to the institution as a alum.