Isopentane

Isopentane, C5H12, also called methylbutane or 2-methylbutane, is a branched-chain alkane with five carbon atoms. Isopentane is an extremely volatile and extremely flammable liquid at room temperature and pressure. The normal boiling point is just a few degrees above room temperature and isopentane will readily boil and evaporate away on a warm day. Isopentane is commonly used in conjunction with liquid nitrogen to achieve a liquid bath temperature of -160 °C.

Nomenclature
Isopentane is the name recommended by the International Union of Pure and Applied Chemistry (IUPAC) in its 1993 Recommendations for the Nomenclature of Organic Chemistry. It is one of only four acyclic hydrocarbons to retain its pre-IUPAC name.

Isomers
Isopentane is one of three structural isomers with the molecular formula C5H12, the others being pentane and neopentane.

Reference

 * 1)  http://www.acdlabs.com/iupac/nomenclature/93/r93_679.htm