The parent compound of a group of organic heterocyclic compounds containing the indole nucleus, which is a benzene ring fused to a pyrrole ring as in indole itself. Indole can exist in two tautomeric forms, the more stable enamine form and the 3-H-indole or imine form. Unsubstituted 3-H-indoles (sometimes called indolenines) and a structural isomer of indole, isoindole, are not stable, but have been shown to be reaction intermediates. They are isolable when properly substituted.