A triplet is a set of three items, and includes in particular:

• one of three babies in a multiple birth
• a preparation of opal as a gemstone, with a thin layer of opal backed with a dark material and covered with cap of clear quartz
• in poetry, a tercet (three lines) with a common rhyme
• in music, a tuplet with a duration two-thirds (2/3) the regular note value duration in simple and/or duple meter. An irrational rhythm, it may also be used as a polyrhythm when played against the regular duration. In relation to triple time duple meter rhythms would be duples.
• in genetics, three nucleotide pairs that form a codon
• in quantum mechanics, a spin triplet is a set of three quantum states of a system, each with total spin S = 1. The system could consist of a single spin 1 W or Z boson, two identical spin 1/2 fermions, or more than two particles in a state with a total spin of 1 (such as the electrons in an oxygen molecule).