Patricia F. O'Malley's Starfleet Academy diploma
Migleemo holding his diploma after graduation
A diploma was a document signifying one's graduation from an educational program.
In 2374, within the Vic's lounge holoprogram, after hearing that Odo, thanks to Vic Fontaine's tutelage, was the head of one's class (i.e. very talented at singing), Melissa continued the metaphor by offering to help him get his diploma. (DS9: "His Way")
During a conversation with Lisa Cusak over subspace communication, Miles O'Brien commented on the common assumption that one could only receive counseling from someone who had a diploma in that field, rather than, as Cusak suggested, one's friends. (DS9: "The Sound of Her Voice")