Hydrocarbons (and by extension, derivatives formed by substitution) having two double bonds from one carbon atom to two others R2C=C=CR2. (The simplest member, propadiene, is known as allene).
See also: cumulenes, dienes
PAC, 1995, 67, 1307 (Glossary of class names of organic compounds and reactivity intermediates based on structure (IUPAC Recommendations 1995)) on page 1314