Twins

Definition
noun, singular: twin
(1) Two siblings from the same pregnancy and birth
(2) (general) Two closely related and resembling entities
Supplement
Twins refer to the two offspring that are brought forth by the same pregnancy and birth. Twins may be monozygotic (or identical) or dizygotic (fraternal). Monozygotic twins are twins that develop from one zygote and splits, forming two embryos. Dizygotic twins are twins that develop from two zygotes. Dizygotic twins are non-identical because they arise from two eggs, each fertilized by a sperm.
Twins may be classified according to the degree of separation in utero: (1) dichorionic-diamniotic twins, (2) monochrorionic-diamniotic twins, (3) monochorionic-monoamniotic twins, and (4) conjoined twins. In dichorionic-diamniotic twins, the twins have separate chorions and amniotic sacs (hence, the name). This is very common among dizygotic twins. This type also has the lowest mortality risk. In monochorionic-diamniotic twins, the twins share the same placenta and (mostly) are diamniotic, i.e. having two amniotic sacs. This is common in monozygotic twins (i.e. 60-70%).¹ In monochorionic-monoamniotic twins, the twins share the same chorion and amniotic sac in utero. In conjoined twins, the identical twins are joined in utero and share one common chorion, placenta, and amniotic sac.
In botany, there is a plant referred to as twinflower. It is a delicate evergreen plant (Linnaea borealis) of northern climates, which has pretty, fragrant, pendulous flowers borne in pairs on a slender stalk.
See also:

• zygote
• ovum
• sperm cell
• fertilization
• Twin pregnancy
• Twin placenta
• Monozygotic Twin
• Identical Twin

Reference(s):

¹ Curran, Mark (2005-11-02). “Twinning”. Focus Information Technology. Retrieved 2008-10-10.