(slang) Siblings born within twelve months of each other. / Used other than figuratively or idiomatically: see Catholic, twin.

The two main buildings of the World Trade Center, destroyed in terrorist attacks on 9/11, 2001.

Being the size of a twin bed.

Either of a pair of matching single beds. / (US) A single bed.

(idiomatic) A duplicate or counterpart that acts in a contrary, nefarious, or insidious manner.

(rare, slang, offensive) Either of a pair of siblings born less than 12 months apart, especially if born within the same calendar year or school year or born one year apart.

plural of Irish twin

(relativity) The clock paradox.

(mathematics) Either of a pair of prime numbers that differ by two.

plural of identical twin