Alkene

[Kai]

any gas in a group that contains carbon and hydrogen atoms, with the carbon atoms joined together in a double bond :

[Dave Paprocki]

any of a group of substances that contain only the elements carbon and hydrogen, which are held together by a chemical bond in which two atoms share two pairs of electrons

[Marquetta]

Any of the series of unsaturated hydrocarbons containing a double bond, including ethylene and propylene.