plēnĭlūnĭum

I. the full moon (post-Aug. for luna plena), Col. 11, 2, 85: cum vero occidente sole orietur ex adverso, ita ut pariter aspiciantur; tunc erit plenilunium, Plin. 18, 32, 75, § 323; in plur., id. 9, 15, 20, § 53.