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Definition of Triplet
Medically reviewed by Min Clinic Staff | Updated: January 2026
Triplet: One of three, as:
1. One of three babies in a multiple birth. Or three animals gestated together.
2. In genetics, a set of three bases in DNA. Also called a codon. See: Crick-Brenner experiment.
3. In optics, a combination of three simple lenses cemented together.
The word triplet came into use in the mid-17th century. It was formed from triple, just as doublet was formed from double.