When mammals have two babies at once, each of them is a twin. If you have a twin brother or sister, the two of you are always the same age.

There are identical twins, who have the same DNA, and fraternal twins, who are more like two siblings who happen to be born at the same time. It's more common to describe humans as twins than other animals. The adjective twin can also describe a pair of things, like the twin chocolate cakes you serve at your birthday party or the "Twin Cities" in Minnesota, Minneapolis and St. Paul.

Definitions of twin

n either of two offspring born at the same time from the same pregnancy

dizygotic twin, fraternal twin
either of two twins who developed from two separate fertilized eggs
identical twin, monozygotic twin, monozygous twin
either of two twins developed from the same fertilized ovum (having the same genetic material)
Siamese twin, conjoined twin
one of a pair of identical twins born with their bodies joined at some point
Type of:
sib, sibling
a person's brother or sister

n a duplicate copy

counterpart, similitude
Type of:
duplicate, duplication
a copy that corresponds to an original exactly

adj being two identical

duplicate, matching, twinned
going well together; possessing harmonizing qualities

v give birth to twins

Type of:
bear, birth, deliver, give birth, have
cause to be born

v grow as twins

twin crystals”
Type of:
increase in size by natural process

v bring two objects, ideas, or people together

couple, match, mate, pair
give or join in marriage
provide with an unsuitable mate
match badly; match two objects or people that do not go together
Type of:
bring together, join
cause to become joined or linked

v duplicate or match

“The polished surface twinned his face and chest in reverse”
duplicate, parallel
Type of:
agree, check, correspond, fit, gibe, jibe, match, tally
be compatible, similar, or consistent; coincide in their characteristics

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