A consonant cluster is a linguistic term, simply meaning a group of consonants which have no intervening vowel. In English, the group spr- is a consonant cluster in the word spring, and the group -nks also is, at the end of the word thinks.

Many languages do not permit consonant clusters at all. Maori and Pirahã, for instance, don't permit any more than one consonant in a row before another vowel can turn up. Japanese is almost as strict, but it allows clusters of n + consonant: Honshu, the name of one of the major islands of Japan, is an example. A great many of the languages of the world are more restrictive than English in terms of consonant clusters: almost every Pacific island nation's language permits either one-term clusters or slight variations on a theme. Tahitian, Fijian, Samoan and Hawaiian are all of this sort. Standard Arabic does not permit initial consonant clusters, or more than two consecutive consonants in other positions. Most spoken dialects, however, are more permissive.

At the other end of the scale, the Kartvelian languages of Georgia are almost unbelievable in terms of the consonant clusters they permit. Clusters are noted in Georgian of four, five or six terms are not unusual - for instance, brt'q'eli (flat), mc'vrtneli (trainer) and prckvna (peeling) - and if grammatical affixes and a flight of the imagination are used, it allows anthropomorphised turkeys to produce an eight-term cluster: gvbrdγvnis (he's plucking us).

Some linguists argue that consonant clusters should be restricted to those that occur within one syllable: English split is an example of this. Others believe that consonant clusters are more useful as a definition when they may occur across syllable boundaries: the Georgian gvbrdγvnis is an example of this type, containing four syllables, but only one vowel.

Consonant clusters occurring in loanwords do not necessarily follow the borrowing language's cluster limits. The Ubykh language's root psta, a loan from Adyghe, violates Ubykh's rule of no more than two initial consonants; also, the English words sphere, sphinx, phthisis and phthalic, all Greek loans, violate the restraint that two fricatives may not appear adjacently word-initially.

In English, the longest possible initial cluster is three terms, as in split; the longest possible final cluster is usually three, as in inks, but for some accents the word infarcts has a four-term final cluster. The word twelfths also has a four-term final cluster.