(covers information from several alternate timelines)
The term twins refers to two siblings born from the same pregnancy. Three siblings born at one time are referred to as triplets, while four are referred to as quadruplets. These siblings can look alike or different and were often close in age being born only moments apart.
When Beverly Crusher was eight years old, she had a crush on an older child named Stefan. As her infatuation with him grew, she began to create the imaginary family the two had together in her head, which consisted of them being married and having three children: twin boys, Andrew and Alexander, and then later, a little girl, Jennifer. (TNG: "The Host")
Twinned Miradorns were not just twin brothers, but together, were a single self: two halves of one being. If one twin died, the other twin would anguish in the loss of self. Ah-Kel and Ro-Kel were an example of a twinning. (DS9: "Vortex")
When Miles and Keiko O'Brien were considering having their second child, Miles asked "What do you say we try for twins..." Keiko told him that "I don't think that's the way it works" and that "you'd better brush up on your biology." (DS9: "Accession")
In Tom Paris' adaption of The Doctor's holonovel Photons Be Free, where Seven of Nine was originally envisioned as Three of Eight, became re-envisioned a group of triplets, including one named Two of Three. (VOY: "Author, Author")
Background information Edit
Identical twins were prominently used in casting the episode "I, Mudd". The use of these twins for each android "series" aided the photographic-effects budget for the episode. With imaginative use of twins and split screens, as many as six of one model were shown at once, while two of the same model required nothing but an additional costume. This ultimately gave the illusion of a planet of thousands of androids. (The Star Trek Compendium)
In the Pocket TOS novel Savage Trade, it was revealed that Vulcan twins shared a platonic mental connection that prevented them from developing a mating bond with another Vulcan. They could therefore have a relationship with whomever they wished.