The butyl group corresponds to the group C₄H₉. The name comes from butyric acid, an acid that has the smell of rancid butter. This fact gives rise to its name, i.e., butyl comes from the Latin word butyrum, meaning butter.

There are two possible arrangements of the atoms in the butyl group, which means there are two isomers: n-butyl and isobutyl. With n-butyl, all four carbon atoms lie in a straight chain, i.e., CH₃CH₂CH₂CH₂-; in the case of isobutyl, there are three carbon atoms in a straight chain with the remaining one being joined to the central carbon atom, i.e., (CH₃)₂CHCH₂.

The butyl group is found in a number of organic compounds, e.g., n-butyl [CH₃CH₂CH₂CH₂OH] and isobutyl [(CH₃)2CHCH₂OH] alcohol, n- butylamine [CH₃CH₂CH₂CH₂NH₂] and isobutyamine [(CH₃)2CHCH₂NH₂], and n-butyl aldehyde [CH₃CH₂CH₂CH₂CHO] and isobutyl aldehyde [(CH₃)2CHCH₂CHO. The characteristics of these groups are determined mainly by the functional group. The presence and size of the butyl group modifies such physical properties as the melting and boiling points of chemical compounds.

Butyl rubber is a synthetic rubber made by co-polymerizing 2-methylpropene and methyl-1,3-diene. This rubber can be vulcanized (treated under heat and pressure with sulfur to improve elasticity and strength). It has been extensively used in the manufacture of inner tubes for cars and bicycles.