Carbenes in which the carbenic carbon atom has a double bond to another carbon atom $\ce{R2C=C{:}}$, e.g. $\ce{H2C=C{:}}$ ethenylidene.
Source: PAC, 1995, 67, 1307. (Glossary of class names of organic compounds and reactivity intermediates based on structure (IUPAC Recommendations 1995)) on page 1374 [Terms] [Paper]