"action or process of infiltrating," in physics, 1796, noun of action from infiltrate. Figurative sense of "a passing into" (anything immaterial) is from 1840; military sense of "stealthy penetration of enemy lines" dates from 1930. The same word had been used earlier in a medical sense of "a knitting together" (early 15c.), from Medieval Latin infiltratio.
early 15c., "fact of intermitting, temporary pause," from Latin intermissionem (nominative intermissio) "a breaking off, discontinuance, interruption," noun of action from past participle stem of intermittere "to leave off, leave an interval," from inter "between" (see inter-) + mittere "let go, send" (see mission). Meaning "lapse of time between events" is from 1560s; specifically of performances (originally plays, later movies, etc.) from 1854.
Intermission is used in U.S. for what we call an interval (in a musical or dramatic performance). Under the influence of LOVE OF THE LONG WORD, it is beginning to infiltrate here and should be repelled; our own word does very well. [H.W. Fowler, "Modern English Usage," 1926]