The isomer $\ce{HN+\!#C^{-}}$ of hydrocyanic acid, $\ce{HC#N}$, and its hydrocarbyl derivatives $\ce{RNC}$ ($\ce{RN^{+}\!#C^{-}}$).
Source: PAC, 1995, 67, 1307. (Glossary of class names of organic compounds and reactivity intermediates based on structure (IUPAC Recommendations 1995)) on page 1344 [Terms] [Paper]