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external (adj.)[external 词源字典]: early 15c., from Middle French externe or directly from Latin externus "outside, outward" (from exterus; see exterior) + -al (1). This version won out over exterial. Related: Externally.[external etymology, external origin, 英语词源]
ad extra: "With effect or result in the outside world (as opposed to one's own mind, self, etc.); in the world at large; externally, perceivably; especially ( Theology ) with reference to an action or characteristic of God which is communicable to the world beyond the Persons of the Trinity. Opposed to ad intra", Early 17th cent.; earliest use found in Thomas Morton (d. ?1647), colonist in America. From post-classical Latin ad extra towards the outside from classical Latin ad to, towards + extrā.
cryotron: "A switching device that depends for its operation on the fact that an externally applied magnetic field destroys superconductivity", 1950s. From cryo- + -tron.