One of the mysteries of the English language finally explained.
Definition of ad extra in English:
ad extra
adverb
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.
adjective
Designating an action that has an effect or result in the outside world; exteriorized, perceivable, external; (Theology) designating or relating to an action or characteristic of God which is communicable beyond the Persons of the Trinity. Opposed to ad intra.
Origin
Early 17th century; 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.