Conjoined twins in the context of "Twin"

Polycephaly is the condition of having more than one head. The term is derived from the Greek stems poly (Greek: "πολύ") meaning "many" and kephalē (Greek: "κεφαλή") meaning "head". A polycephalic organism may be thought of as one being with a supernumerary body part, or as two or more beings with a shared body.

Two-headed animals (called bicephalic or dicephalic) and three-headed (tricephalic) animals are the only type of multi-headed creatures seen in the real world, and form by the same process as conjoined twins from monozygotic twin embryos.

Giacomo and Giovanni Battista Tocci were conjoined twins born in Locana, Italy, between 1875 and 1877 in either July or October. They toured in a circus and retired about 1900; details of their later lives are uncertain.

Supernumerary body parts are most commonly a congenital disorder involving the growth of an additional part of the body and a deviation from the body plan. Body parts may be easily visible or hidden away, such as internal organs.

Many additional body parts form by the same process as conjoined twins: the zygote begins to split but fails to completely separate. This condition may also be a symptom of repeated occurrences of continuous inbreeding in a genetic line.

In linguistics and stylistics, an irreversible binomial, frozen binomial, binomial freeze, binomial expression, binomial pair, or nonreversible word pair is a pair of words, used together, in fixed order, as an idiomatic expression or collocation. The words typically have a semantic relationship, usually involving the word and or or. They also belong to the same part of speech: for example: nouns (e.g., milk and honey); adjectives (short and sweet); or verbs (do or die). Usually (as with the preceding examples) the order of such word elements will not be reversed.

The term "irreversible binomial" was introduced by Yakov Malkiel in 1954, though various aspects of the phenomenon had been discussed, since at least 1903, under different names: a "terminological imbroglio". Ernest Gowers used the name Siamese twins (i.e., conjoined twins) in the 1965 edition of Fowler's Modern English Usage. The 2015 edition reverts to the scholarly name, "irreversible binomials", as "Siamese twins" had become "politically incorrect".