mid-15c., "an inhabitant, one who dwells in a place permanently or for a considerable time," from resident (adj.). Meaning "medical graduate doing supervised specialized practice in a hospital as training" is attested by 1892, American English.
adjectival word-forming element, variant of -al (1) with connective -i-. From Latin -ialis, in which the -i- originally was from the stem of the word being attached but later came to be felt as connective.
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Harper, D. (n.d.). Etymology of residential. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/residential