'astromancer' originates from Modern English, formed by combining 'astro-' (from Greek 'astron' meaning 'star') and the suffix '-mancer' (from Greek 'manteia' meaning 'divination').

'-mancer' is patterned on older formations such as 'necromancer' (via Latin and Old French forms deriving from Greek μαντεία 'manteia'), and 'astromancer' is a later English coinage using that suffix with 'astro-'.

Initially it meant 'one who practices astromancy' (a star-based diviner); over time—especially in modern and fantasy contexts—it has also come to mean 'a wielder of star-based magic' or 'star-powered wizard'.