The Wittig reaction is a chemical reaction used to form alkenes by reacting an aldehyde or ketone with a phosphonium ylide. In this process, the ylide acts as a nucleophile, attacking the carbonyl carbon of the aldehyde or ketone, leading to the formation of an intermediate that ultimately eliminates a phosphine oxide to yield the alkene. This reaction is significant in organic chemistry because it allows for the synthesis of alkenes with specific configurations and substituents. The Wittig reaction is widely used in the preparation of complex molecules, including natural products and pharmaceuticals, due to its efficiency and versatility.