diplomo

The etymology is the best thing about this word, so take it first. Greek diploos means double, a diploma is a thing folded in half, and that is all a diploma ever was: an official letter folded over. The same fold gives us the diplomat, who was originally simply the person carrying the folded credentials. Two English words, one piece of stationery.

Now the seam that is not there. Esperanto has -om at the end of a whole row of correlatives - kiom, tiom, ĉiom, iom, neniom, all of them about quantity - and it also has the fraction suffix -on- you met on the plafono page. Neither is in this word. Diplom- is one Greek root, and there is no dipl- to build it from.

The useful derivations are diplomiĝi, to graduate, literally to become diploma'd, and diplomito, a graduate - the passive participle again, the person who was awarded one. A diplomo is also not automatically a university degree: for the academic rank you want grado, and a diplomo can be the certificate from a weekend course as easily as from a doctorate. If you need to say which, say which.