GALERUS or GALERUM, was originally a covering for the head worn by priests, especially by the flamen dialis (Gell. x.15; Serv. ad Virg. Aen. ii.683). It appears to have been a round cap made of leather, with its top ending in an apex or point. The word is probably connected with galea, a helmet. In course of time the name was applied to any kind of cap fitting close to the head like a helmet (Virg. Aen. vii.688; Virg. Moret. 121; Suet. Ner. 26). Galerus and its diminutive Galericulum are also used to signify a covering for the head made of hair, and hence a wig (Juv. Sat. vi.120, with the Schol.; Suet. Oth. 12; Mart. xiv.50).

William Smith, A Dictionary of Greek and Roman Antiquities, John Murray, London, 1875. p 566.