Interlunium, interlunij. n. g. Plin. The space when neither the old moone nor the new is seene, the coniunction of the suune and moone.
Lewis and Short: Latin dictionary
interlūnĭum, ĭi, n. [interlunis], the new moon, time of new moon, interlunary interval, Plin. 18, 32, 75, 324; 7, 5, 4, 38. —In plur., Hor. C. 1, 25, 11.