Banana is the compound known as amyl acetate, which is the condensation product of 1-pentanol (five carbon atoms, C5H11OH) and acetic acid. The alcohol 1-pentanol is one of eight pentanols that are known as amyl alcohol. It is also a component of fusel alcohol.
Truth be told, a lot of British strains are POF+ (phenolic off-flavor positive) and throw amyl acetate when the conditions are right (a few throw sulfur as well). These compounds are usually a sign of stress.