Illustrated Glossary of Organic Chemistry

Butene: Any of the four isomers of an alkene with exactly four carbon atoms.

1-Butene
A terminal monosubstituted alkene
   
2-Methyl-1-propene (isobutylene)
A terminal disubstituted alkene




Cis-2-butene (Z-2-butene)
An internal disubstituted alkene
   
Trans-2-butene (E-2-butene)
An internal disubstituted alkene