Underground culture, or just underground, is a term to describe various alternative cultures which either consider themselves different to the mainstream of society and culture, or are considered so by others.
The term "The underground" was a common name for World War II resistance movements. It was later applied to counter-cultural movement(s) many of which sprang up during the 1960s.
These 1960s and 1970s underground cultural movements had some connections to the "beat generation" which had, in turn, been inspired by the philosophers, artists and poets of the Paris Existentialist movement.
Jack Kerouac (In Esquire magazine in 1958)  said: "The same thing was almost going on in the postwar France of Sartre and Genet and what's more we knew about it--But as to the actual existence of a Beat Generation, chances are it was really just an idea in our minds--We'd stay up 24 hours drinking cup after cup of black coffee, playing record after record of Wardell Gray, Lester Young, Dexter Gordon, Willie Jackson, Lennie Tristano and all the rest, talking madly about that holy new feeling out there in the streets- -We'd write stories about some strange beatific Negro hepcat saint with goatee hitchhiking across Iowa with taped up horn bringing the secret message of blowing to other coasts, other cities, like a veritable Walter the Penniless leading an invisible First Crusade- -We had our mystic heroes and wrote, nay sung novels about them, erected long poems celebrating the new 'angels' of the American underground--In actuality there was only a handful of real hip swinging cats and what there was vanished mightily swiftly during the Korean War when (and after) a sinister new kind of efficiency appeared in America, maybe it was the result of the universalization of Television and nothing else (the Polite Total Police Control of Dragnet's 'peace' officers) but the beat characters after 1950 vanished into jails and madhouses, or were shamed into silent conformity, the generation itself was shortlived and small in number." It took a few years more, however, for the culture Kerouac describes to grow in numbers and redefine itself variously as the underground culture or the freak scene etc.
Applied to the arts, the term underground typically means artists that are not corporately sponsored and generally do not want to be.
Perhaps the best way to define it is a quote by Frank Zappa:
- "The mainstream comes to you, but you have to go to the underground."