Dimethylamine $\left[\left( CH _{3}\right)_{2} NH \right]$ is a secondary amine as the central Nitrogen atom is substituted by two methyl groups and the nitrogen has one hydrogen. Tert-butyl amine, secondary butyl amine, isobutylamine are all the isomers of $n$-butyl amine, varying in the position of the $N$ - group in the alkyl chain.
