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In morpheme-based morphology, a null morpheme is a morpheme that is realized by a phonologically null affix (an empty string of phonological segments). In simpler terms, a null morpheme is an "invisible" affix. It's also called zero morpheme; the process of adding a null morpheme is called null affixation, null derivation or zero derivation. The concept was first used over two thousand years ago by Pāṇini in his Sanskrit grammar. (Some linguists object to the notion of a null morpheme, since it sets up (they say) an unverifiable distinction between a "null" or "zero" element, and nothing at all.)

The null morpheme is represented as either the figure zero (0), the empty set symbol ø, or its variant Ø.

Examples in English include hiatus and co-operation.

The existence of a null morpheme in a word can also be theorized by contrast with other forms of the same word showing alternate morphemes. For example, the singular number of English nouns is shown by a null morpheme that contrasts with the plural morpheme -s.

• cat = cat + -Ø = ROOT ("cat") + SINGULAR
• cats = cat + -s = ROOT ("cat") + PLURAL

In addition, there are some cases in English where a null morpheme indicates plurality in nouns that take on irregular plurals.

• sheep = sheep + -Ø = ROOT ("sheep") + SINGULAR
• sheep = sheep + -Ø = ROOT ("sheep") + PLURAL

Also, a null morpheme marks the present tense of verbs in all forms but the third person singular:

• (I) run = run + -Ø = ROOT ("run") + PRESENT: Non-3rd-SING
• (He) runs = run + -s = ROOT ("run") + PRESENT: 3rd-SING

According to some linguists' view, it's also a null morpheme that turns some English adjectives into verbs of the kind of to clean, to slow, to warm. Null derivation, also known as conversion if the word class changes, is very common in English.

In languages that show the above distinctions, it's quite common to employ null affixation to (not) mark singular number, present tense and third persons (English is unusual in its marking of the third person singular with a non-zero morpheme, by contrast with a null morpheme for others). It's also frequent to find null affixation for the least-marked cases (the nominative in nominative-accusative languages, and the absolutive in ergative-absolutive languages).

In most languages of the world these are the affixes that are realized as null morphemes. But in some cases roots may alse be realized as these. For instance, Russian word вы-Ø-ну-ть (vynut', to take out) consists of one prefix (вы-), one zero root (-Ø-), one suffix (-ну-), and one postfix (-ть).

A basic radical element plus a null morpheme is not the same as an uninflected word, though usage may make those equal in practice.

• Affix
• Hiatus (linguistics)
• Lemma (linguistics)
• Linguistics
• Marker (linguistics)
• Morphology (linguistics)
• Root (linguistics)
• Uninflected word
• Zero (linguistics)