Grad is an informal short way to say graduate: a person who has finished a course of study, most often high school, college, or university. Americans say “a college grad,” “a recent grad,” “high-school grads.” It also sits in front of other words: grad school (the school you go to after a first degree), grad student (a student there), grad program. Though actually, that second pattern can trip people up — “a grad” by itself usually means someone who already finished, but a “grad student” is still studying for a master’s or a PhD. British English uses graduate more often and grad a little less, though British students know it from US films and from “grad school.” It is campus talk and casual writing; you would not put grad on a formal diploma. Older people use it too, but it still sounds like school and work-life chat rather than a ceremony word.