A triplet lens is an optical

Optics

Optics is the branch of physics which involves the behavior and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behavior of visible, ultraviolet, and infrared light...

 device consisting of three single lenses

Lens (optics)

A lens is an optical device with perfect or approximate axial symmetry which transmits and refracts light, converging or diverging the beam. A simple lens consists of a single optical element...

. The triplet design is the simplest to give the required number of degrees of freedom

Degrees of freedom (physics and chemistry)

A degree of freedom is an independent physical parameter, often called a dimension, in the formal description of the state of a physical system...

 to allow the lens designer to overcome all Seidel aberrations.

The term is used in two ways. The three lenses may be cemented together, as in the Steinheil triplet. Or a triplet may be designed with three spaced glasses, e.g. the Cooke triplet

Cooke triplet

The Cooke triplet is a photographic lens designed and patented in 1893 by Dennis Taylor who was employed as chief engineer by T. Cooke & Sons of York...

.