foramen

Foramen, foraminis is third declension neuter, from foro, "to bore, pierce", with the -men suffix that Latin uses to name the result of an action: what boring produces is a foramen. Compare flumen from fluo, lumen from luceo, and semen from sero.

Being neuter, nominative and accusative are the same in both numbers, and the plural foramina is the form you meet most, since holes rarely come singly. A foramen is specifically a hole bored or pierced through something solid, as against an ostium, which is a doorway, or a hiatus, which is a gap that has opened up. Anatomy took the word wholesale and never gave it back: the foramen magnum at the base of the skull, and several dozen smaller ones, all keep the Latin plural.