An elusive concept used in the context of criminal law to describe a publication which is illegal because it is morally corruptive.
The common law has struggled with this word as society has evolved towards greater tolerance of alternative sexual behavior.
Historically, it included any lewd material which had no apparent social value, which was offensive to contemporary community standards of decency, and even material which tended to invoke impure sexual thoughts.
As an example of a modern definition, Canada has defined obscene material as any publication a dominant characteristic of which is the undue exploitation of sex, or of sex and crime, horror, cruelty or violence.