Doublet is:

• a Doublet (clothing), a man's snug-fitting buttoned jacket that was worn in medieval and Tudor times
• a Doublet (lapidary), a fake gem composed in two sections, such as a garnet overlaying green glass
• a Doublet (lens), a type of lens, made up of two stacked layers with different refractive indices
• a Doublet (linguistics), one of two or more words of the same language that come from the same root
• a Doublet (physics), a quantum state of a system with a spin of 1/2
• a derivative of the Dirac delta function