Lewis Short
plēnĭlūnĭum (noun N) : (sc. tempus) [plenus-luna]
* 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.
Charlton T. Lewis, Charles Short, A Latin Dictionary