What’s the word to describe if you’re not a twin (born solo)?

Celine Dion is now the proud momma of twin boys. We’re happy for the entertainment diva, and pleased at the excuse to explore the language of twins.

Dion’s boys are not identical twins. They’re fraternal. This means that they derived from separately fertilized ova and that they have different genetic makeup. Fraternal twins may be of the same or of the opposite sex.

While some fraternal twins resemble each other, some look as different as second cousins. Only time will tell which physical features the singer’s twins will have in common.

Naturally, identical twins do closely resemble each other and are always of the same sex. Identical twins develop from a single fertilized ovum and share the same genotype — meaning they share the same genetic stuff. The degree of shared DNA is called zygosity.

If you’re not a twin, then it’s likely that you’re a singleton. Card players are likely familiar with the term. A singleton is also a card that is the only one of a suit in a hand. It is also a phrase used to describe chronically single young people popularized by “Bridget Jones’s Diary.”

Siamese twins are congenitally united twins. The term is derived from a famous pair of conjoined twins named Chang and Eng who lived from 1811 to 1874. The twins were born in Siam, now named Thailand, and were joined at the chest.

Conjoined twins are so rare that statistics aren’t available to measure their incidence. Taken together, all types of twins make up about 1.9% of the world population.