Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
plēnĭlūnĭum, ii, 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.