Alkene
Definition
Hydrocarbons with at least one carbon-carbon double bond: more reactive than alkanes, and the building blocks of many plastics.
Technical definition
Alkenes are unsaturated hydrocarbons containing at least one C=C double bond (general formula CnH2n for one double bond). The pi bond makes them reactive toward addition reactions, and they are the monomers for polymers like polyethylene.
Example
Ethene (C2H4) is ripening fruit's signaling gas and the monomer of polyethylene plastic.
Common confusion
The double bond is not twice as strong in a simple additive way. The pi bond is weaker and more exposed than the sigma bond, which is exactly why alkenes react at the double bond.