In linguistics, a zero is a constituent needed in analysis but not realized in speech. Specifically in phonology, it refers to an element that is phonologically null. This implies that there is a lack of an element where a theory would expect one. It is usually written with the symbol "∅", in Unicode U+2205 ∅ Empty set (HTML ∅ · ∅). A common ad hoc solution is to use the Scandinavian capital letter Ø instead.
There are several kind of zeros.
A zero morph, consisting of no phonetic form, is an allomorph of a morpheme that is otherwise realized in speech. In the phrase two sheep-∅, the plural marker is a zero morph, which is an allomorph of -s as in two cows. In the phrase I like-∅ it, the verb conjugation has a zero affix, as opposed to the third-person singular present -s in he likes it.
A zero pronoun occurs in some languages. In the English sentence nobody knows ∅ the zero pronoun plays the role of the object of the verb, and in ∅ makes no difference it plays the role of the subject. Likewise, the zero pronoun in the book ∅ I am reading plays the role of the relative pronoun that in the book that I am reading. This is also referred to as PRO. In pronoun-dropping languages, including null subject languages such as most Romance languages, the zero pronoun is a prominent feature.
A zero subordinate conjunction occurs in English in sentences like I know ∅ he likes me, in which the zero conjunction plays the role of the subordinate conjunction that in I know that he likes me.
A zero article is an unrealized indefinite or definite article in some languages.
A zero copula, in which a copula such as the verb to be is implied but absent. For example, in Russian the copula is usually omitted in the present tense, as in "Она красивая" (literally: She beautiful), the same happening with colloquial Brazilian Portuguese, as in "irônicos, aqueles" (literally: ironic, those [guys]), though never with the adjective coming after the subject as usual in Romance languages. In English the copula is sometimes omitted in some nonstandard dialects.
