Operetta is a form of theatre and a genre of light opera. It includes spoken dialogue, songs and including dances. It is lighter than opera in terms of its music, orchestral size, and length of the work. Apart from its shorter length, the operetta is usually of a light and amusing character. The subject matter may portray "lovers' spats, mistaken identities, sudden reversals of fortune, and glittering parties". It sometimes also includes satirical commentaries.

"Operetta" is the Italian diminutive of "opera" and was used originally to describe a shorter, perhaps less ambitious work than an opera. Operetta provides an alternative to operatic performances in an accessible form targeting a different audience. Operetta became a recognizable form in the mid-19th century in France, and its popularity led to the development of many national styles of operetta. Distinctive styles emerged across countries including Austria-Hungary, Germany, England, Spain, the Philippines, Mexico, Cuba, and the United States. Through the transfer of operetta among different countries, cultural cosmopolitanism emerged in the previous century. Operetta as a genre lost favor in the 1930s and gave way to modern musical theatre. Important operetta composers include Johann Strauss, Jacques Offenbach, Franz Lehár, and Francisco Alonso.

The term operetta arises in the mid-eighteenth-century Italy and it is first acknowledged as an independent genre in Paris around 1850. Castil-Blaze's Dictionnaire de la musique moderne claims that this term has a long history and that Mozart was one of the first people to use the word operetta, disparagingly, describing operettas as "certain dramatic abortions, those miniature compositions in which one finds only cold songs and couplets from vaudeville". The definition of operetta has changed over the centuries and ranges depending on each country's history with the genre. It is often used to refer to pieces that resemble the one-act compositions by Offenbach in contrast with his full length compositions, 'opéra-bouffe'. Offenbach invented this art form in response to the French government's oppressive laws surrounding the stagings of works that were larger than one act or contained more than four characters.

Operetta became recognized as a musical genre around 1850 in Paris. In 1870, the centre for operetta shifted to Vienna when Paris fell to the Prussians. The form of operetta continued to evolve through the First World War.

There are some common characteristics among operettas that flourished from the mid-1850s through the early 1900s, beginning with the French opéra-bouffe. They contain spoken dialogue interspersed between musical numbers, and often the principal characters, as well as the chorus, are called upon to dance, although the music is largely derived from 19th-century operatic styles, with an emphasis on singable melodies. Operetta in the twentieth century is more complex and reached its pinnacle in Austria and Germany.

Operetta is a precursor of the modern musical theatre or the "musical". In the early decades of the 20th century, operetta continued to exist alongside the newer musicals, with each influencing the other. The distinctive traits of operetta are found in the musical theatre works of Jerome Kern, Richard Rodgers and Stephen Sondheim.

Operetta was first created in Paris, France in the middle of the 19th century in order to satisfy a need for short, light works in contrast to the full-length entertainment of the increasingly serious opéra comique. By this time, the "comique" part of the genre name had become misleading: Georges Bizet's Carmen (1875) is an example of an opéra comique with a tragic plot. The definition of "comique" meant something closer to "humanistic", meant to portray "real life" in a more realistic way, representing tragedy and comedy next to each other, as Shakespeare had done centuries earlier. With this new connotation, opéra comique had dominated the French operatic stage since the decline of tragédie lyrique. The origins of French operetta began when comic actors would perform dances and songs to crowds of people at fairs on open-air stages. In the beginning of the 18th century these actors began to perform comic parodies of known operas. These performances formed operetta as a casual genre derived from opéra comique, while returning to a simpler form of music. Many scholars have debated as to which composer should be credited as the inventor of operetta; Jaques Offenbach or Hervé. It is concluded that Hervé completed the groundwork, and Offenbach refined and developed the art form into the concept of operetta as we know it today. Therefore, "Offenbach is considered the father of French operetta – but so is Hervé."

Hervé was a singer, composer, librettist, conductor, and scene painter. In 1842, he wrote the one act opérette, L'Ours et le pacha, based on the popular vaudeville by Eugène Scribe and X. B. Saintine. In 1848, Hervé made his first notable appearance on the Parisian stage, with Don Quichotte et Sancho Pança (after Cervantes), which can be considered the starting point for the new French musical theatre tradition. Hervé's most famous works are the Gounod parody Le petit Faust (1869) and Mam'zelle Nitouche (1883).