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 optics a compound lens which is made from three components in proximity to each other (or fastened together)
• 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 elementary spin 1 particle such as the W or Z bosons, or be a multiparticle state built out of arbitrary other spin states with total spin one.
• in mathematics, a triplet is a tuple of length 3.
• a tandem bicycle with three seats
• in photography or paint art, a triplet is a set of three photos.