The isocyanic acid tautomer, $\ce{HN=C=O}$, of cyanic acid, $\ce{HOC≡N}$ and its hydrocarbyl derivatives RN=C=O.
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]