Serbo-Croatian, also known as Bosnian-Croatian-Montenegrin-Serbian (BCMS), is a South Slavic language and the primary language of Serbia, Croatia, Bosnia and Herzegovina, and Montenegro. It is a pluricentric language with four mutually intelligible standard varieties, namely Serbian, Croatian, Bosnian, and Montenegrin.

South Slavic languages historically formed a dialect continuum. The region's turbulent history, particularly due to the expansion of the Ottoman Empire, led to a complex dialectal and religious mosaic. Due to population migrations, Shtokavian became the most widespread supradialect in the western Balkans, encroaching westward into the area previously dominated by Chakavian and Kajkavian. Bosniaks, Croats, and Serbs differ in religion and were historically often part of different cultural spheres, although large portions of these populations lived side by side under foreign rule. During that period, the language was referred to by various names, such as "Slavic" in general, or "Serbian", "Croatian" or "Bosnian" in particular. In a classicizing manner, it was also referred to as "Illyrian".

The standardization of Serbo-Croatian was initiated in the mid-19th-century Vienna Literary Agreement by Croatian and Serbian writers and philologists, decades before a Yugoslav state was established. From the outset, literary Serbian and Croatian exhibited slight differences, although both were based on the same Shtokavian dialect—Eastern Herzegovinian. In the 20th century, Serbo-Croatian served as the lingua franca of the country of Yugoslavia, being the sole official language in the Kingdom of Yugoslavia (when it was called "Serbo-Croato-Slovenian"), and afterwards the official language of four out of six republics of the Socialist Federal Republic of Yugoslavia. The breakup of Yugoslavia influenced language attitudes, leading to the ethnic and political division of linguistic identity. Since then, Bosnian has likewise been established as an official standard in Bosnia and Herzegovina, and efforts to codify a separate Montenegrin standard continue.

Like other South Slavic languages, Serbo-Croatian has a relatively simple phonology, with the common five-vowel system and twenty-five consonants. Its grammar evolved from Common Slavic, with complex inflection, preserving seven grammatical cases in nouns, pronouns, and adjectives. Verbs exhibit imperfective or perfective aspect, with a moderately complex tense system. Serbo-Croatian is a pro-drop language with flexible word order, subject–verb–object being the default. It can be written in either the Latin (Gaj's Latin alphabet) or Cyrillic script (Serbian Cyrillic alphabet), and the orthography is highly phonemic in all standards. Despite the many linguistic similarities among the standard varieties, each possesses distinctive traits, although these differences remain minimal.

Throughout the history of the South Slavs, the vernacular, literary, and written languages (e.g., Chakavian, Kajkavian, Shtokavian) of various regions and ethnic groups developed and diverged independently. Before the 19th century, these languages were collectively called "Illyrian", "Slavic", "Slavonian", "Bosnian", "Dalmatian", "Serbian", or "Croatian". Since the 19th century, the term Illyrian or Illyric was frequently used, sometimes leading to confusion with the ancient Illyrian language. Although the word Illyrian was used occasionally before, its widespread usage began after Ljudevit Gaj and several other prominent linguists met at Ljudevit Vukotinović's house to discuss the issue in 1832. The term Serbo-Croatian was first used by Jacob Grimm in 1824, later popularized by the Viennese philologist Jernej Kopitar, and adopted by Croatian grammarians in Zagreb in 1854 and 1859. At that time, Serb and Croat lands were still part of the Ottoman and Austrian Empires.

Serbo-Croatian is typically referred to by the names of its standardized varieties—Serbian, Croatian, Bosnian, and Montenegrin. It is rarely referred to by the names of its sub-dialects, such as Bunjevac or Šokac. In the language itself, it is formally known as srpskohrvatski, српскохрватски ("Serbo-Croatian") and hrvatskosrpski, хрватскoсрпски ("Croato-Serbian"). Historically, linguists and philologists, including Đuro Daničić and Tomislav Maretić, have referred to the language as "Serbian or Croatian" and "Croatian or Serbian". Serbo-Croatian is often colloquially called naš jezik ("our language") or naški (sic. "ourish" or "ourian") by native speakers. This term is frequently used by those who wish to avoid linguistic discussions. Native speakers traditionally describe their language as jedan ali ne jedinstven ("one but not uniform").

In 1988, Croatian linguist Dalibor Brozović advocated the term Serbo-Croatian, stating that, by analogy with Indo-European, it not only denotes the two components of the same language but also delineates the geographical region in which it is spoken, encompassing all language varieties within these boundaries, including Bosnian and Montenegrin. Croatian linguist Mate Kapović suggested Standard Shtokavian as the ethnically neutral and linguistically most precise term. Nowadays, the use of the term "Serbo-Croatian" is controversial due to the widespread perception that national identity and language should correspond. However, it is still used in academic and linguistic contexts due to the lack of a succinct alternative. Following the breakup of Yugoslavia, alternative designations have emerged, such as Bosnian/Croatian/Serbian (BCS), which is frequently used in political contexts, including by the International Criminal Tribunal for the former Yugoslavia.

In the mid-19th century, Serbian (led by self-taught writer and folklorist Vuk Stefanović Karadžić) and most Croatian writers and linguists (represented by the Illyrian movement and led by Ljudevit Gaj and Đuro Daničić), proposed the use of the most widespread dialect, Shtokavian, as the base for their common standard language. Karadžić standardised the Serbian Cyrillic alphabet, and Gaj and Daničić standardized the Croatian Latin alphabet, on the basis of vernacular speech phonemes and the principle of phonological spelling. In 1850 Serbian and Croatian writers and linguists signed the Vienna Literary Agreement, declaring their intention to create a unified standard. Thus a complex bi-variant language appeared, which the Serbs officially called "Serbo-Croatian" or "Serbian or Croatian" and the Croats "Croato-Serbian", or "Croatian or Serbian". Yet, in practice, the variants of the conceived common literary language served as different literary variants, chiefly differing in lexical inventory and stylistic devices. The common phrase describing this situation was that Serbo-Croatian or "Croatian or Serbian" was a single language. In 1861, after a long debate, the Croatian Sabor put up several proposed names to a vote of the members of the parliament; "Yugoslavian" was opted for by the majority and legislated as the official language of the Triune Kingdom. The Austrian Empire, suppressing Pan-Slavism at the time, did not confirm this decision and legally rejected the legislation, but in 1867 finally settled on "Croatian or Serbian" instead. During the Austro-Hungarian rule in Bosnia and Herzegovina, the language of all three nations in this territory was declared "Bosnian" until the death of administrator von Kállay in 1907, at which point the name was changed to "Serbo-Croatian".