Know the words

Twin Glossary: Words for Doubles

Twin science and the wider world of doubles come with a lot of jargon. Here are the key words used across Actual Twin, explained in plain English.

Black and white diagram of a womb holding two small embryos inside one shared amniotic sac and one shared chorion

Identical twins sharing one sac, a setup linked to later splits: Altheahayton, CC BY-SA 3.0

A to Z

Word	Meaning
Amnion	The inner sac that holds a baby and the fluid around it. Some identical twins share one.
Barycentre	The point two orbiting objects circle together, like the spot in space between Pluto and Charon.
Binary star	Two stars orbiting each other.
Chimera	A living thing carrying cells from two different individuals, as freemartin calves often do.
Chorion	The outer membrane around a developing baby that helps form the placenta.
Clone	A living thing that is genetically identical to another, such as an identical twin or a plant grown from a cutting.
Conjoined twins	Identical twins whose bodies stay physically joined.
Cryptophasia	A private language invented by young twins, often called twin talk.
Dizygotic	Formed from two fertilised eggs. The scientific word for fraternal twins.
Doppelganger	An unrelated person who looks remarkably like someone else.
Double planet	A pair of worlds of similar size orbiting each other, like Pluto and Charon.
Enclave	A piece of one country completely surrounded by another, like the Belgian pieces of Baarle.
Freemartin	A heifer born twin to a bull calf, usually unable to have calves.
Heritability	The share of differences in a trait within a group that can be linked to genetic differences.
Mimicry	When one species evolves to look like another, like the viceroy butterfly and the monarch.
Mirror-image twins	Identical twins with reversed features, such as one right-handed and one left-handed.
Monozygotic	Formed from one fertilised egg. The scientific word for identical twins.
Polyembryony	One fertilised egg always splitting into several embryos, as in nine-banded armadillos.
Sesquizygotic	Semi-identical twins, formed from one egg and two sperm.
Sister city	A city that has a formal friendship with a city in another country, also called a twin town.
Superfecundation	When two eggs released in the same cycle are fertilised at different times.
Tidal locking	When a moon always shows the same face to its planet. Pluto and Charon are locked to each other.
Zygote	The single cell formed when an egg and a sperm join.