The term avant-garde has been used historically since the 15th century to mean artists or works of art that are particularly innovative or ahead of general trends. The pioneering spirit characterizes many of the innovations in art since the Renaissance, and therefore what is now considered traditional may have once been avant-garde. In this sense the term is difficult to apply in a parallel manner across the history of art. However, it has been associated with several periods, and in particular work of the 20th century. There is an element of needing to disturb in avant-garde art, and so, an atmosphere of traditionalism or stagnancy needs to exist before the avant-garde can arise. Before the end of World War II, the term was widely used. It is thought, however, that the popularity of the New York Abstract Expressionists provided such an atmosphere of acceptance that since then, little has been considered avant-garde, and in its place, post-modern has been used. This aside, British art of the 1990s may have once again established an avant-garde. Charles Saatchi's privately owned art collection was displayed first in 1997 at the Royal Academy under the title “Sensation.” The exhibition gained immediate notoriety and a fair share of infamy. "Sensation" featured many young British artists, exploring themes that some considered morbid, distasteful, and even blasphemous. Journalists had no choice but to regard the exhibition as avant-garde; the work itself was adventurous, and the response from the public and the art community, while mixed, was certainly passionate.