What defines an alkene structurally?

Alkenes are defined structurally by the presence of at least one double carbon-carbon bond (C=C) in their molecular structure. This characteristic is what distinguishes alkenes from alkanes, which contain only single carbon-carbon bonds, and alkynes, which possess triple carbon-carbon bonds. The double bond in alkenes not only affects their chemical properties, making them more reactive than alkanes, but it also influences isomerism and the types of reactions they undergo, such as addition reactions. Therefore, the defining feature of alkenes is indeed the presence of a double carbon-carbon bond, making the answer correct.