Final answer:
The prefix for an isomer with two similar or identical groups on the same side of a double bond is 'cis-'. Cis-trans isomerism occurs in compounds with double bonds such as alkenes and in ring structures like cycloalkanes.
Step-by-step explanation:
The prefix used for an isomer in which two similar or identical groups are on the same side of a double bond is cis-. This term is derived from the Latin word 'cis', meaning 'on this side of'. In chemistry, cis-trans isomerism is a form of stereoisomerism where isomers have the same type and sequence of bonded atoms but differ in the orientation of these atoms in space due to the presence of a double bond or a ring structure, which prevents free rotation.
For example, in geometric isomers of alkenes such as cis-2-butene and trans-2-butene, the methyl groups in the cis isomer are on the same side of the double bond, while in the trans isomer, they are on opposite sides. A similar situation exists for cycloalkanes, where the two groups can either be on the same side (cis) or on opposite sides (trans) of the ring.