Bossa nova (Portuguese pronunciation: [ˈbɔsɐ ˈnɔvɐ] ^ⓘ) is a relaxed style of samba developed in the late 1950s and early 1960s in Rio de Janeiro, Brazil. It is mainly characterized by a calm syncopated rhythm with chords and fingerstyle mimicking the beat of a samba groove, as if it was a simplification and stylization on the guitar of the rhythm produced by a samba school band. Another defining characteristic of the style is the use of unconventional chords in some cases with complex progressions and "ambiguous" harmonies. A common misconception is that these complex chords and harmonies were derived from jazz, but samba guitar players have been using similar arrangement structures since the early 1920s, indicating a case of parallel evolution of styles rather than a simple transference from jazz to bossa nova. Nevertheless, bossa nova was influenced by jazz, both in the harmonies used and also by the instrumentation of songs, and today many bossa nova songs are considered jazz standards. The popularity of bossa nova has helped to renew samba and contributed to the modernization of Brazilian music in general.

One of the major innovations of bossa nova was the way to synthesize the rhythm of samba on the classical guitar. According to musicologist Gilberto Mendes, the bossa nova was one of the "three rhythmic phases of samba", in which the "bossa beat" had been extracted by João Gilberto from the traditional samba. The synthesis performed by Gilberto's guitar was a reduction of the "batucada" of samba, a stylization produced from one of the percussion instruments: the thumb stylized a surdo; the index, middle and ring fingers phrased like a tamborim. In line with this thesis, musicians such as Baden Powell, Roberto Menescal, and Ronaldo Bôscoli also understand the bossa nova beat as being extracted from the tamborim play in the bateria.

In Brazil, the word bossa is old-fashioned slang for something done with particular charm, natural flair or innate ability. As early as 1932, Noel Rosa used the word in a samba:

O samba, a prontidão e outras bossas são nossas coisas, são coisas nossas.
(Samba, readiness and other bossas are our things, are things from us.)

The phrase bossa nova, translated literally, means "new trend" or "new wave" in Portuguese. The exact origin of the term bossa nova remained unclear for many decades, according to some authors. Within the artistic beach culture of the late 1950s in Rio de Janeiro, the term bossa was used to refer to any new "trend" or "fashionable wave". In his book Bossa Nova, Brazilian author Ruy Castro asserts that bossa was already in use in the 1950s by musicians as a word to characterize someone's knack for playing or singing idiosyncratically.

Castro claims that the term bossa nova might have first been used in public for a concert given in 1957 by the Grupo Universitário Hebraico do Brasil ('Hebrew University Group of Brazil'). The authorship of the term bossa nova is attributed to the young journalist Moyses Fuks, who was promoting the event. That group consisted of Sylvia Telles, Carlos Lyra, Nara Leão, Luiz Eça, Roberto Menescal, and others. Mr Fuks's description, fully supported by most of the bossa nova members, simply read "HOJE. SYLVIA TELLES E UM GRUPO BOSSA NOVA" ("Today. Sylvia Telles and a 'Bossa Nova' group"), since Sylvia Telles was the most famous musician in the group at that time.

In 1959, Nara Leão also participated in more than one embryonic display of bossa nova. These include the 1st Festival de Samba Session, conducted by the student union of Pontifícia Universidade Católica. This session was chaired by Carlos Diegues (later a prominent Cinema Novo film director), a law student whom Leão ultimately married.

The nightclubs of Beco das Garrafas [pt] (Alley of the Bottles), a small dead-end alley in Copacabana, is considered a historical cradle of bossa nova.