Plenilune is, prosaically, the full moon or the time of a full moon.
In a letter to his aunt in 1961, J R R Tolkien wrote of this word that it was beautiful even before it was understood, that he wished he could have the pleasure of meeting it for the first time again, and that “Surely the first meeting should be in a living context, and not in a dictionary.”
Sadly, that is unlikely, its having dropped almost entirely out of use. Even coming across it in dictionaries would be unlikely, as only the very largest include it these days. But then it has always been poetic and literary, from Ben Jonson’s “Whose glory (like a lasting Plenilune) / Seems ignorant of what it is to wane” of 1601, down to James Joyce’s “What counsel has the hooded moon / Put in thy heart, my shyly sweet, / Of love in ancient plenilune, / Glory and stars beneath his feet” in Chamber Music in 1907.
Tolkien employed it in The Adventures of Tom Bombadil, published in 1962: “Of crystal was his habergeon, / his scabbard of chalcedony; / with silver tipped at plenilune / his spear was hewn of ebony.” [Habergeon: A sleeveless coat or jacket of mail or scale armour.] A rare recent sighting is in William Weaver’s translation of Umberto Eco’s Island of the Day Before (1995): “You can see ... when recur the Sundays and the Epacts, and the Solar Circle, and the Moveable and Paschal Feasts, and novilunes and plenilunes, quadratures of the sun and moon.”