hypocorism
Hypocorism (noun) means an affectionate name for someone, often in diminutive form; a pet name.
Etymology
Borrowed from Late Latin hypocorisma, itself a loan from Ancient Greek ὑποκόρισμα (hupokórisma), ὑποκορισμός (hupokorismós, "pet name"), which came from ὑποκορίζεσθαι (hupokorízesthai), ὑποκορίζομαι (hupokorízomai, "to use a pet name; to act in a childish manner"), from ῠ̔πο- (hŭpo-, a prefix indicating a small degree) and κορίζομαι (korízomai, "to caress") (in turn from κόρος (kóros, "boy; youth") and κόρη (kórē, "girl; young woman")).