In the context of organic molecules, aryl is any functional group or substituent derived from an aromatic ring, usually an aromatic hydrocarbon, such as phenyl and naphthyl. "Aryl" is used for the sake of abbreviation or generalization, and "Ar" is used as a placeholder for the aryl group in chemical structure diagrams, analogous to "R" used for any organic substituent. "Ar" is not to be confused with the elemental symbol for argon.
The most basic aryl group is phenyl, which is made up of a benzene ring with one hydrogen atom substituted for some substituent, and has the molecular formula C6H5−. Note that phenyl groups are not the same as benzyl groups, which consists of a phenyl group attached to a methyl group, and has the molecular formula C6H5CH2−.
To name compounds containing phenyl groups, the phenyl group can be taken to be the parent hydrocarbon and being represented by the suffix "-benzene". Alternatively, the phenyl group could be treated as the substituent, being described within the name as "phenyl". This is usually done when the group attached to the phenyl group consists of six or more carbon atoms.
As an example, consider a hydroxyl group connected to a phenyl group. In this case, if the phenyl group was taken as the parent hydrocarbon, the compound would be named hydroxybenzene. Alternatively, and more commonly, the hydroxyl group could be taken as the parent group (and the phenyl group treated as the substituent), resulting in the more familiar name phenol.