In linguistics, a broken plural (or internal plural) is an irregular plural form of a noun or adjective found in the Semitic languages and other Afroasiatic languages such as Berber. Broken plurals are formed by changing the pattern of consonants and vowels inside the singular form. They contrast with sound plurals (or external plurals), which are formed by adding a suffix. It is distinct from Germanic umlaut, a form of vowel mutation found in Germanic languages.
While the phenomenon is known from several Semitic languages, nowhere has it become as productive as in Arabic.
In Arabic, the regular way of making a plural for a masculine noun is adding the suffix -ūn (for the nominative) or -īn (for the accusative and genitive) at the end. For feminine nouns, the regular way is to add the suffix -āt. However, not all plurals follow these simple rules. One class of nouns in both spoken and written Arabic produce plurals by changing the pattern of vowels inside the word, sometimes also with the addition of a prefix or suffix. This system is not fully regular, and it is used mainly for masculine non-human nouns. Human nouns are pluralized regularly or irregularly.
Broken plurals are known as jam‘ taksīr (جَمع تَكسير, literally "plural of breaking") in Arabic grammar. These plurals constitute one of the most unusual aspects of the language, given the very strong and highly detailed grammar and derivation rules that govern the written language. Broken plurals can also be found in languages that have borrowed many words from Arabic, for instance Persian, Pashto, Turkish, Kurdish, Azerbaijani and Urdu, and sometimes exist in both a broken plural Arabic form and a local, adapted plural, e.g. in Pashto where the word for purpose (مطلب) can be pluralised in either its Arabic form مطالب for more formal, High Pashto, or the according to Pashto rules of plural as مطلبونه in everyday speech.
In Persian this kind of plural is called jameh mokassar (جَمِع مُکَسَر, literally "broken plural").
Full knowledge of these plurals can come only with extended exposure to the language, though a few rules can be noted.
Semitic languages typically form triconsonantal roots, forming a "grid" into which vowels may be inserted without affecting the basic root.
Here are a few examples; note that the commonality is in the root consonants (capitalized), not the vowels.
- KiTāB كِتَاب "book" → KuTuB كُتُب "books"
- KāTiB كَاتِب "writer, scribe" → KuTTāB كُتَّاب "writers, scribes"
- maKTūB مَكْتُوْب "letter" → maKāTīB مَكَاتِيْب "letters"
- note: these three words all have a common word root, K-T-B ك – ت – ب "to write"
- WaLaD وَلَد "boy" → ʼaWLāD أَوْلَاد "boys"
- WaRaQah وَرَقَة "paper" → ʼaWRāQ أَوْرَاق "papers"
- ŠaJaRah شَجَرَة "tree" → ʼaŠJāR أَشْجَار "trees"
- but: JaMaL جَمَل "camel" → JiMāL جِمَال "camels"
- maKTaB مَكَتَب "desk, office" → maKāTiB مَكَاتِب "offices"
- maLBaS مَلْبَس "dress, garb" → maLāBiS مَلَابِس "clothes"
- JaDD جَدّ "grandfather" → JuDūD جُدُوْد or aJDāD أَجْدَاد "grandfathers"
- FaNN فَنّ "art" → FuNūN فُنُوْن "arts"
- but: RaBB رَبّ "master, owner, lord" → ʼaRBāB أَرْبَاب "masters"
Patterns in Arabic 
|CiCāC||CuCuC||كتاب||kitāb (book)||كتب||kutub (books)|
|CaCīCa||سفينة||safīna (ship)||سفن||sufun (ships)||juzur (islands),
|CaCīC||سبيل||sabīl (path)||سبل||subul (paths)|
|CuCCa||CuCaC||غرفة||ġurfa (room)||غرف||ġuraf (rooms)|
|CaCCa||شقة||šaqqa (apartment)||شقق||šuqaq (apartment)|
|CiCC||CiCaC||قط||qiṭṭ (cat)||قطط||qiṭaṭ (cats)|
|CaCC||CuCūC||قلب||qalb (heart)||قلوب||qulūb (hearts)||funūn (arts),
|CiCC||علم||ʻilm (science)||علوم||ʻulūm (sciences)|
|CuCC||جحر||juḥr (hole)||جحور||juḥūr (holes)|
|CaCC||CiCāC||كلب||kalb (dog)||كلاب||kilāb (dogs)|
|CiCC||ظل||ẓill (science)||ظلال||ẓilāl (sciences)|
|CuCC||رمح||rumḥ (spear)||رماح||rimāḥ (spears)|
|CaCuC||رجل||rajul (man)||رجال||rijāl (men)|
|CaCC||ʼaCCāC||يوم||yawm (day)||أيام||ʼayyām (days)|
|CiCC||حلم||ḥilm (prudence)||أحلام||ʼaḥlām (meaning minds)|
|CuCC||ربع||rubʻ (quarter)||أرباع||ʼarbāʻ (quarters)||ʼaʻmāq (deeps)|
|CaCaC||سبب||sabab (cause)||أسباب||ʼasbāb (causes)||ʼawlād (boys),
|CaCūC||ʼaCCiCah||عمود||ʻamūd (pole)||أعمدة||ʼaʻmidah (poles)||Ends with taʼ marbutah|
|CaCīC||ʼaCCiCāʼ||صديق||ṣadīq (friend)||أصدقاء||ʼaṣdiqāʼ (friends)|
|CaCīC||CuCaCā'||سعيد||saʻīd (happy)||سعداء||suʻadāʼ(happy)||wuzarāʼ (ministers)||mostly for adjectives and occupational nouns|
|CāCiC||CuC2C2āC||كاتب||kātib (writer)||كتاب||kuttāb (writers)||ṭullāb (students)||Gemination of the second root; mostly for occupational nouns|
|CāCiCa||CawāCiC||قائمة||qāʼima (list)||قوائم||qawāʼim (lists)||bawārij (battleships)|
|CāCūC||CawāCīC||صاروخ||ṣārūḫ (rocket)||صواريخ||ṣawārīḫ (rockets)||ḥawāsīb (computers)|
|CiCāCa||CaCāʼiC||رسالة||risāla (message)||رسائل||rasāʼil (messages)|
|CaCīCa||جزيرة||jazīra (island)||جزائر||jazāʼir (islands)||haqāʼib (suitcases),
|CaCCaC||CaCāCiC||دفتر||daftar (notebook)||دفاتر||dafātir (notebooks)||applies to all four-literal nouns with short second vowel|
|CuCCuC||فندق||funduq (hotel)||فنادق||fanādiq (hotels)|
|maCCaC||maCāCiC||ملبس||malbas (apparel)||ملابس||malābis (apparels)||makātib (offices)||Subcase of previous, with m as first literal|
|maCCiC||مسجد||masjid (mosque)||مساجد||masājid (mosques)||manāzil (houses)|
|miCCaCa||منطقة||minṭaqa (area)||مناطق||manāṭiq (areas)|
|CvCC(ā/ī/ū)C||CaCāCīC||صندوق||ṣandūq (box)||صناديق||ṣanādīq (boxes)||applies to all four-literal nouns with long second vowel|
|miCCāC||maCāCīC||مفتاح||miftāḥ (key)||مفاتيح||mafātīḥ (keys)||Subcase of previous, with m as first literal|
|maCCūC||مكتوب||maktūb (message)||مكاتيب||makātīb (messages)|
In Hebrew, though all plurals must take either the sound masculine (-im ים-) or feminine (-ot ות-) plural suffixes, the historical stem alternations of the so-called segolate or consonant-cluster nouns between CVCC in the singular and CVCaC in the plural have often been compared to broken plural forms in other Semitic languages. Thus the form malkī מלכי "my king" in the singular is opposed to məlaxīm מלכים "kings" in the plural.
In addition, there are many other cases where historical sound changes have resulted in stem allomorphy between singular and plural forms in Hebrew (or between absolute state and construct state, or between forms with pronominal suffixes and unsuffixed forms etc.), though such alternations do not operate according to general templates accommodating root consonants, and so are not usually considered to be true broken plurals by linguists.
See also 
- Elative (gradation)
- Triconsonantal root
- Nonconcatenative morphology
- Apophony#Apophony versus transfixation (root-and-pattern)
- "Ge'ez (Axum)" by Gene Gragg in The Cambridge Encyclopedia of the World's Ancient Languages edited by Roger D. Woodard (2004) ISBN 0-521-56256-2, p. 440.
- "Hebrew" by P. Kyle McCarter Jr. in The Cambridge Encyclopedia of the World's Ancient Languages edited by Roger D. Woodard (2004) ISBN 0-521-56256-2, p. 342.