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).
Source:
PAC, 1995, 67, 1307
(Glossary of class names of organic compounds and reactivity intermediates based on
structure (IUPAC Recommendations 1995))
on page 1314