A typological universal, also known as a Greenberg universal, is a general statement of a systematic pattern across the structures of languages, such as in their morphology (word structure), syntax (word arrangement) or phonology. Much research in linguistic typology follows from the work of Joseph H. Greenberg, who identified a series of patterns in the languages of the world and which were more frequent.
'Universal' may not only mean 'in all languages', but 'in all instances of a single language', i.e. the domain in which the rule applies may be across all languages, in a subset of languages, or in a single language (effectively, smaller and smaller 'universes'). For example, the statement that If an English word begins with three consonants, the first is /s/ is a typological universal within English, and makes a prediction that is true (there are no English words such as *ktmar). The same rule cannot be applied to all other languages, however, so is not a true linguistic universal, but is an implicational universal (see below). Also, relatively few cross-linguistic universal statements constitute hard and fast rules, but more are overwhelmingly true across languages.
Generalisations may be unidirectional or bidirectional, i.e. the patterns may or may not entail each other. The above example is unidirectional because it is not true to say that if an English word begins with /s/, then it must begin with three consonants (or e.g. sip would be ruled out). Another universal, this time bidirectional, is that if the verb follows the object, e.g. in Japanese keeki-o taberu 'eats cake', the language is very likely to have postpositions (where the modifying particle follows the content word, e.g. Japanese Tookyoo e 'to Tokyo') and not prepositions (where the particle precedes the word it modifies, as in English). In other words, if we find postpositions, we will probably find verb-final phrases, and if we find prepositions, we can predict that the verb will be initial.
The best-known type of linguistic generalisation is the implicational universal, where a structure in one part of the grammar of a language entails that a similar pattern will be repeated elsewhere in that language and possibly in other languages, typically expressed through an if-then statement. An example is If a language has voiced obstruents [such as /b/ or /v/], then it will have voiceless obstruents, e.g. /p/ or /s/. To discover this, it is necessary to survey a wide variety of languages, i.e. it is not entailed automatically by some external factor such as the shape of the articulators. (As it stands, this generalisation is indeed unidirectional: there are many languages which have voiceless obstruents only, such as Hawaiian.) The above example about consonant clusters in English is also a unidirectional implicational universal, this time applying to just one language.
Other uses of 'universal'
Typological universals are similar to but distinguished from both linguistic universals and universal tendencies, and the term is often confused with use of 'universal' elsewhere in linguistics.
The use of the term typological universal is not quite the same as linguistic universal (also referred to as an absolute linguistic universal). The latter would be a statement that applies to all languages, such as All languages have verbs.
Typological universals are also not the same as a universal tendencies. These are patterns which are frequently found but may not make any implicational claim. For example, the majority of languages have nasals, but some do not.
The word universal should also not be taken to refer to universal grammar, which is a theory of language popularly associated with Noam Chomsky, and deals with a far more abstract and underlying structure which is responsible for generating all possible grammars. It does not really address the superficial typological patterns found in the small set of natural languages which actually exist.