In linguistics, a zero or null is a segment which is not pronounced or written. It is a useful concept in analysis, indicating lack of an element where one might be expected. It is usually written with the symbol "∅", in Unicode U+2205 ∅ EMPTY SET (∅, ∅, ∅, ∅). A common ad hoc solution is to use the Scandinavian capital letter Ø instead.

There are several kinds of zero:

• In phonetics and phonology, a null phoneme or zero phone indicates that no phone is produced where one might be expected. For example, in syllable structure analysis, null onset indicates that a syllable lacks an initial consonant (onset) that is normally required by phonotactics of the considered language. For an example, see Standard Chinese phonology#Zero onset.
• In morphology, a zero morph,^[1] 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 (see nouns with identical singular and plural forms), 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.
• In grammar, a zero pronoun occurs in some languages.^[2] 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. In generative grammar, 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, such as the plural indefinite article in English.
• A zero copula,^[3] 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'). In English the copula is sometimes omitted in some nonstandard dialects.^[4]

• Empty string
• Zero consonant
• Silent letter
• Zero-marking in English
• Zero-marking language

• What is a zero?, SIL Glossary of linguistic terms