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.