Why are vicinal dihalides named for halogens on adjacent carbon atoms?

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organohalogen compound Vinyl chloride is an organohalogen compound that has important industrial applications. When treated with certain catalysts, vinyl chloride monomers undergo polymerization and form the larger compound known as polyvinyl chloride, or PVC. PVC is used in the manufacture of numerous products, including packaging films and water pipes.

The term "vicinal" is derived from the Latin word vicinalis, which means "neighboring" or "neighbor". In the context of organohalogen compounds, this etymology is applied to describe chemical substituents that occupy adjacent sites within a molecule.1

Therefore, a vicinal dihalide is specifically named for having two halogen atoms attached to neighboring, or adjacent, carbon atoms. This distinguishes them from geminal dihalides, where both halogen atoms are substituted on the same carbon atom.2