hemi·acetal \| ̷ ̷ ̷ ̷ at hemi-+\noun Etymology:hemi- (I) + acetal : any of a class of compounds characterized by the grouping >C(OH)(OR) and usually formed as intermediate products in the preparation of acetals from aldehydes or ketones < an aldose is a favorable compound for the study of hemiacetals — C.D.Hurd > — see glycoside, monosaccharide