I examine Jacques Maritain's definition of art and what it contributes to debates on definitions of art in contemporary aesthetics. Of particular concern are the so-called ‘fine arts’. I make three claims; first, that Maritain's definition of art is superior to other traditional and contemporary theories in avoiding their pitfalls and accommodating key data to be met by definitions of art, such as the existence of avant-garde art. Second, I claim that in accommodating avant-garde art, in particular ‘conceptual art’, Maritain's definition of art refutes the Wittgensteinian argument that art is an ‘open concept’ which cannot be defined. My final, resulting claim is that Maritain's definition deserves to be the pre-eminent definition of art today. This article consists of four sections. Section one provides some context of definitions of art and the data and conditions that have to be met by a viable definition. Section two provides the foundation and first pillar of Maritain's definition of art, what I identify as a cumulative ‘descriptive’ definition. Section three provides the second pillar of Maritain's definition of art, applicable specifically to the fine arts. Section four applies Maritain's full descriptive definition to the problem of conceptual art.