Derivatives of diazene (diimide), $\ce{HN=NH}$, wherein both hydrogens are substituted by hydrocarbyl groups, e.g. $\ce{PhN=NPh}$ azobenzene or diphenyldiazene.
Source: PAC, 1995, 67, 1307. (Glossary of class names of organic compounds and reactivity intermediates based on structure (IUPAC Recommendations 1995)) on page 1321 [Terms] [Paper]