Illustrated Glossary of Organic Chemistry

Allene: A molecule having two carbon-carbon double bonds that share a single sp carbon. Also refers to propadiene (H2C=C=CH2), the parent member of this series.

Molecular Structure of Allene
Note the perpendicular pi bonds, and perpendicular terminal methylene groups.

Lewis structure
Ball and spoke
Space-filling model
Molecular model kit

2,3-Pentadiene, a typical allene