A secondary amine is an organic compound that contains an amino group (-NH2) bonded to two carbon atoms. The general formula for secondary amines is R2NH. An example of a secondary amine among the given options is di-butylamine, which has the formula (C4H9)2NH. Propylene is not a secondary amine since it does not contain an amino group. Methylamine is a primary amine, and trimethylamine is a tertiary amine since it has three alkyl groups attached to the nitrogen atom.