A prefix meaning “the same” as in such terms as isomer (the same part), isotope (the same place), isometric (the same measure), isobar (the same pressure), etc. In organic chemistry, it denotes an isomer of an alkyl hydrocarbon, alcohol, etc., having a subordinate chain of one or more carbon atoms attached to a carbon of the straight chain.See Isobutane; Isooctane; branched chain.