The Collaborative International Dictionary of English v.0.48:

Hydrazine \Hy"dra*zine\, n. [Hydr- + azo- + -ine.] (Chem.)
   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.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Amidogen \A*mid"o*gen\, n. [Amide + -gen.] (Chem.)
   A compound radical, NH2, not yet obtained in a separate
   state, which may be regarded as ammonia from the molecule of
   which one of its hydrogen atoms has been removed; -- called
   also the amido group, and in composition represented by the
   form amido.
   [1913 Webster]