Modernism has pretty solid parameters. The movement started in the late 19th century. Artists and the movement were influenced by and railed against industrialization. Impressionism in painting and sculpture and symbolism in poetry and writing also found its place in modernism. However, it wasn’t until the lead-up to the First World War and the social unrest that it brought, that Modernism really found its footing. While Manet and Monet are usually acknowledged as part of the Modernist movement, the rise of Picasso and Matisse a few decades after its start, provide the aesthetics many most closely-associate with the style today.
The modernist style was created to push against tradition in order to find new ways to experience art, to think about the world and portray the human condition. Ironically, today modern art could be considered part of tradition. It’s generally accepted that the modern art movement ended in the 1970s.