n.
  - one of two children or animals born at the same birth.
- something containing or consisting of two matching or corresponding parts.
adj.
   - forming or being one of a twin.
- forming a matching or closely connected pair.
- (of a crystal) twinned.
v.
 (twins, twinning, twinned)
  - link; combine. ▸Brit. link (a town or district) with another in a different country, for the purposes of cultural exchange. 
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[as adj. twinned]
 (of a crystal) that is a composite consisting of two (or more) parts which are reversed in orientation with respect to each other.
Derivative
  
  Etymology
  OE twinn ‘double’, from twi- ‘two’; rel. to ON tvinnr.