In chemistry, the term geminal (from Latin gemini = twins) refers to the relationship between two functional groups that are attached to the same atom. The prefix gem is applied to a chemical name to denote this relationship, as in a gem-dibromide.
The following example shows the conversion of a cyclohexyl methyl ketone to a gem-dichloride through a reaction with phosphorus pentachloride. This gem-dichloride can then be used to synthesize an alkyne.
The term is often used in NMR spectroscopy, as the relationship between atoms is important in determining the size of coupling constants.
The related term vicinal refers to the relationship between two functional groups that are attached to adjacent atoms.