Any one of a series of nitrogenous bases, resembling the
   amines and produced by the reduction of certain nitroso and diazo
   compounds; as, methyl hydrazine, phenyl hydrazine, etc. They are
   derivatives of hydrazine proper, H2N.NH2, which is a doubled amido
   group, recently (1887) isolated as a stable, colorless gas, with a
   peculiar, irritating odor. As a base it forms distinct salts. Called
   also diamide, amidogen, (or more properly diamidogen), etc.