twin 
(twĭn)
(twĭn)n.
1. One of two offspring born at the same birth.
2. One of two identical or similar people, animals, or things; a counterpart.
3. twins Mineralogy Two interwoven crystals that are mirror images of each other.
4. A twin bed.
adj.
1. Being two or one of two offspring born at the same birth: twin sisters.
2. Being two or one of two identical or similar people, animals, or things: twin cities; a twin bed.
3. Botany Of or relating to structures, such as flowers, that occur in pairs.
4. Consisting of two identical or similar parts: a twin lamp fixture.
5. Being of a size that will fit a twin bed: twin sheets.
v. twinned, twin·ning, twins
v.intr.
1.
a. To give birth to twins.
b. Archaic To be one of twin offspring.
2. To be paired or coupled.
v.tr.
1. To pair or couple.
2. To provide a match or counterpart to.
[Middle English, from Old English twinn, twofold; see dwo- in the Appendix of Indo-European roots.]