An art movement that is perhaps the broadest of its kind. In fact you might want to consider it an umbrella term for much thinking and manifestation of thought from the 1900 and onwards. Some would argue it coincided with large-scale adaptation to a thoroughly industrialised society.
Within modernism you find several other movements including the usual suspects expressionism, futurism, cubism, abstract expressionism, lyrical abstraction, and pop art. In art, Modernism rejects the ideology of realism. Therein lies the perhaps biggest clue as to how you’d identify e.g. a modernist painting or sculpture. It is either abstract, semi-abstract or expressive in a way that sets it apart from realism.
Modernism, or the modern, gives rise to much polemic debate given how broad and vague it is. The term is also unfairly problematic given that the general public considers it synonymous with ‘new’ or ‘newness’. It raises the question: what do you call a future spin off, the day the so-called modern ages? Do you call it new-new, modern-modern or post-modern?
Suggestion: See also main article on expressionism