twinning

Concise Oxford English Dictionary © 2008 Oxford University Press:
twin/twɪn/

▶noun

• 1 one of two children or animals born at the same birth.

• 2 something containing or consisting of two matching or corresponding parts.

▶adjective

• 1 forming or being one of a twin.

• 2 forming a matching or closely connected pair.

• 3 (of a crystal) twinned.

▶verb (twins, twinning, twinned)

• 1 link; combine.

  ■ Brit. link (a town or district) with another in a different country, for the purposes of cultural exchange.

• 2 (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.

– derivatives
twinned adjective,
twinning noun.

– origin OE twinn ‘double’, from twi- ‘two’; rel. to ON tvinnr.