As jazz spread around the world, it drew on different national, regional, and local musical cultures, which gave rise to many distinctive styles. New Orleans jazz began in the early 1910s, combining earlier brass-band marches, French quadrilles, biguine, ragtime and blues with collective polyphonic improvisation. In the 1930s, heavily arranged dance-oriented swing big bands, Kansas City jazz, a hard-swinging, bluesy, improvisational style and Gypsy jazz (a style that emphasized musette waltzes) were the prominent styles. Bebop emerged in the 1940s, shifting jazz from danceable popular music toward a more challenging "musician's music" which was played at faster tempos and used more chord-based improvisation. Cool jazz developed near the end of the 1940s, introducing calmer, smoother sounds and long, linear melodic lines.
The 1950s saw the emergence of free jazz, which explored playing without regular meter, beat and formal structures, and in the mid-1950s, hard bop emerged, which introduced influences from rhythm and blues, gospel, and blues, especially in the saxophone and piano playing. Modal jazz developed in the late 1950s, using the mode, or musical scale, as the basis of musical structure and improvisation. Jazz-rock fusion appeared in the late 1960s and early 1970s, combining jazz improvisation with rock music's rhythms, electric instruments, and highly amplified stage sound. In the early 1980s, a commercial form of jazz fusion called smooth jazz became successful, garnering significant radio airplay. Other styles and genres abound in the 2000s, such as Latin and Afro-Cuban jazz.
The origin of the word jazz has resulted in considerable research, and its history is well documented. It is believed to be related to jasm, a slang term dating back to 1860 meaning "pep, energy". The earliest written record of the word is in a 1912 article in the Los Angeles Times in which a minor league baseball pitcher described a pitch which he called a jazz ball "because it wobbles and you simply can't do anything with it".
The use of the word in a musical context was documented as early as 1915 in the Chicago Daily Tribune. Its first documented use in a musical context in New Orleans was in a November 14, 1916 Times-Picayune article about "jas bands". In an interview with NPR, musician Eubie Blake offered his recollections of the original slang connotations of the term, saying: "When Broadway picked it up, they called it 'J-A-Z-Z'. It wasn't called that. It was spelled 'J-A-S-S'. That was dirty, and if you knew what it was, you wouldn't say it in front of ladies." The American Dialect Society named it the Word of the Twentieth Century.
Jazz is difficult to define because it encompasses a wide range of music spanning a period of over 100 years, from ragtime to the rock-infused fusion. Attempts have been made to define jazz from the perspective of other musical traditions, such as European music history or African music. But critic Joachim-Ernst Berendt argues that its terms of reference and its definition should be broader, defining jazz as a "form of art music which originated in the United States through the confrontation of the Negro with European music" and arguing that it differs from European music in that jazz has a "special relationship to time defined as 'swing'". Jazz involves "a spontaneity and vitality of musical production in which improvisation plays a role" and contains a "sonority and manner of phrasing which mirror the individuality of the performing jazz musician". In the opinion of Robert Christgau, "most of us would say that inventing meaning while letting loose is the essence and promise of jazz".
A broader definition that encompasses different eras of jazz has been proposed by Travis Jackson: "it is music that includes qualities such as swing, improvising, group interaction, developing an 'individual voice', and being open to different musical possibilities". Krin Gibbard argued that "jazz is a construct" which designates "a number of musics with enough in common to be understood as part of a coherent tradition". In contrast to commentators who have argued for excluding types of jazz, musicians are sometimes reluctant to define the music they play. Duke Ellington, one of jazz's most famous figures, said, "It's all music."
Although jazz is considered difficult to define, in part because it contains many subgenres, improvisation is one of its key elements. The centrality of improvisation is attributed to the influence of earlier forms of music such as blues, a form of folk music which arose in part from the work songs and field hollers of African-American slaves on plantations. These work songs were commonly structured around a repetitive call-and-response pattern, but early blues was also improvisational. Classical music performance is evaluated more by its fidelity to the musical score, with less attention given to interpretation, ornamentation, and accompaniment. The classical performer's goal is to play the composition as it was written. In contrast, jazz is often characterized by the product of interaction and collaboration, placing less value on the contribution of the composer, if there is one, and performer. The jazz performer interprets a tune in individual ways, never playing the same composition twice. Depending on the performer's mood, experience, and interaction with band members or audience members, the performer may change melodies, harmonies, and time signatures.