Twin (noun)

1. One of two children or animals born at the same birth.
2. Either of two things that are exactly alike or closely similar.
3. Something having two parts or components that are usually joined together.

Origin:

Middle english, from old english twinn; akin to old high german zwīn two, latin duos.

Examples:

1. The twins were identical and looked exactly alike.
2. He had a twin brother who lived across the country.
3. The couple had twin daughters who were born premature.
4. The twin engines provided the necessary power for the plane.
5. She held a twin flower in each hand, one for herself and one for her friend.