A linguistic universal is a pattern that occurs systematically across natural languages, potentially true for all of them. For example, All languages have nouns and verbs, or If a language is spoken, it has consonants and vowels. Research in this area of linguistics is closely tied to the study of linguistic typology, and intends to reveal generalizations across languages, likely tied to cognition, perception, or other abilities of the mind. The field was largely pioneered by the linguist Joseph Greenberg, who derived a set of forty-five basic universals, mostly dealing with syntax (see the list), from a study of some thirty languages.
Linguists distinguish between two kinds of universals: absolute (opposite: statistical, often called tendencies) and implicational (opposite non-implicational). Absolute universals apply to every known language and are quite few in number; an example is All languages have pronouns. An implicational universal applies to languages with a particular feature that is always accompanied by another feature, such as If a language has trial grammatical number, it also has dual grammatical number, while non-implicational universals just state the existence (or non-existence) of one particular feature.
Also in contrast to absolute universals are tendencies, statements that may not be true for all languages, but nevertheless are far too common to be the result of chance. They also have implicational and non-implicational forms. An example of the latter would be The vast majority of languages have nasal consonants. However, most tendencies, like their universal counterparts, are implicational. For example, With overwhelmingly-greater-than-chance frequency, languages with normal SOV order are postpositional. Strictly speaking, a tendency is not a kind of universal, but exceptions to most statements called universals can be found. For example, Latin is an SOV language with prepositions. Often it turns out that these exceptional languages are undergoing a shift from one type of language to another. In the case of Latin, its descendant Romance languages switched to SVO, which is a much more common order among prepositional languages.
Universals may also be bidirectional or unidirectional. In a bidirectional universal two features each imply the existence of each other. For example, languages with postpositions usually have SOV order, and likewise SOV languages usually have postpositions. The implication works both ways, and thus the universal is bidirectional. By contrast, in a unidirectional universal the implication works only one way. Languages that place relative clauses before the noun they modify again usually have SOV order, so pre-nominal relative clauses imply SOV. On the other hand, SOV languages worldwide show little preference for pre-nominal relative clauses, and thus SOV implies little about the order of relative clauses. As the implication works only one way, the proposed universal is a unidirectional one.
Linguistic universals in syntax are sometimes held up as evidence for universal grammar (although epistemological arguments are more common). Other explanations for linguistic universals have been proposed, for example, that linguistic universals tend to be properties of language that aid communication. If a language were to lack one of these properties, it has been argued, it would probably soon evolve into a language having that property.
Michael Halliday has argued for a distinction between descriptive and theoretical categories in resolving the matter of the existence of linguistic universals, a distinction he takes from J.R. Firth and Louis Hjelmslev. He argues that "theoretical categories, and their inter-relations construe an abstract model of language...; they are interlocking and mutually defining". Descriptive categories, by contrast, are those set up to describe particular languages. He argues that "When people ask about 'universals', they usually mean descriptive categories that are assumed to be found in all languages. The problem is there is no mechanism for deciding how much alike descriptive categories from different languages have to be before they are said to be 'the same thing'" 
In the domain of semantics, research into linguistic universals has taken place in a number of ways. Some linguists, starting with Leibniz, have pursued the search for a hypothetic irreducible semantic core of all languages. A modern variant of this approach can be found in the Natural Semantic Metalanguage of Wierzbicka and associates. Other lines of research suggest cross-linguistic tendencies to use body part terms metaphorically as adpositions, or tendencies to have morphologically simple words for cognitively salient concepts. The human body, being a physiological universal, provides an ideal domain for research into semantic and lexical universals. In a seminal study, Cecil H. Brown (1976) proposed a number of universals in the semantics of body part terminology, including the following: in any language, there will be distinct terms for BODY, HEAD, ARM, EYES, NOSE, and MOUTH; if there is a distinct term for FOOT, there will be a distinct term for HAND; similarly, if there are terms for INDIVIDUAL TOES, then there are terms for INDIVIDUAL FINGERS. Subsequent research has shown that most of these features have to be considered cross-linguistic tendencies rather than true universals. Several languages, for example Tidore and Kuuk Thaayorre, lack a general term meaning 'body'. On the basis of such data it has been argued that the highest level in the partonomy of body part terms would be the word for 'person'.
- Dryer (1998)
- Lushootseed and Rotokas are examples of the rare languages which truly lack nasal consonants as normal speech sounds.
- Halliday, M.A.K. 2002. A personal perspective. In On Grammar, Volume 1 in the Collected Works of M.A.K. Halliday. London and New York: Continuumm p12.
- see for example Goddard & Wierzbicka (1994) and Goddard (2002).
- Heine (1997)
- Rosch et al. (1976)
- Wilkins (1993), Enfield et al. 2006:17.
- Brown, Cecil H. (1976) "General principles of human anatomical partonomy and speculations on the growth of partonomic nomenclature." American Ethnologist 3, no. 3, Folk Biology, pp. 400–424
- Comrie, Bernard (1981) Language Universals and Linguistic Typology. Chicago: University of Chicago Press.
- Croft, W. (2002). Typology and Universals. Cambridge: Cambridge UP. 2nd ed. ISBN 0-521-00499-3
- Dryer, Matthew S. (1998) "Why Statistical Universals are Better Than Absolute Universals" Chicago Linguistic Society 33: The Panels, pp. 123-145.
- Enfield, Nick J. & Asifa Majid & Miriam van Staden (2006) 'Cross-linguistic categorisation of the body: Introduction' (special issue of Language Sciences).
- Ferguson, Charles A. (1968) 'Historical background of universals research'. In: Greenberg, Ferguson, & Moravcsik, Universals of human languages, pp. 7–31.
- Goddard, Cliff and Wierzbicka, Anna (eds.). 1994. Semantic and Lexical Universals - Theory and Empirical Findings. Amsterdam/Philadelphia: John Benjamins.
- Goddard, Cliff (2002) 'The search for the shared semantic core of all languages'. In Goddard & Wierzbicka (eds.) Meaning and Universal Grammar - Theory and Empirical Findings volume 1, pp. 5–40, Amsterdam/Philadelphia: John Benjamins.
- Greenberg, Joseph H. (ed.) (1963) Universals of Languages. Cambridge, Mass.: MIT Press.
- Greenberg, Joseph H. (ed.) (1978a) Universals of Human Language Vol. 4: Syntax. Stanford, California: Stanford University Press.
- Greenberg, Joseph H. (ed.) (1978b) Universals of Human Language Vol. 3: Word Structure. Stanford, California: Stanford University Press.
- Heine, Bernd (1997) Cognitive Foundations of Grammar. New York/Oxford: Oxford University Press.
- Song, Jae Jung (2001) Linguistic Typology: Morphology and Syntax. Harlow, UK: Pearson Education (Longman).
- Song, Jae Jung (ed.) (2011) Oxford Handbook of Linguistic Typology. Oxford: Oxford University Press.
- Rosch, E. & Mervis, C.B. & Gray, W.D. & Johnson, D.M. & Boyes-Braem, P. (1976) 'Basic Objects In Natural Categories', Cognitive Psychology 8-3, 382-439.
- Wilkins, David P. (1993) ‘From part to person: natural tendencies of semantic change and the search for cognates’, Working paper No. 23, Cognitive Anthropology Research Group at the Max Planck Institute for Psycholinguistics.
- Some Universals of Grammar with Particular Reference to the Order of Meaningful Elements by Joseph H. Greenberg
- The Universals Archive by the University of Konstanz