Final answer:
A C6H5 group when acting as a substituent is referred to as a phenyl group, which is a benzene ring with one less hydrogen atom allowing it to attach to a carbon chain.
Step-by-step explanation:
When a C6H5 group acts as a substituent in organic chemistry, it is referred to as a phenyl group. This group is essentially a benzene ring, C6H6, with one hydrogen atom removed to allow for attachment to a parent carbon chain. Phenyl groups are common in organic chemistry and have distinct properties and reactions compared to alkyl substituents such as methyl or ethyl groups. The presence of a phenyl group in a compound can significantly alter the compound's physical and chemical behaviors.