The phrase "fluid medium" refers to a substance that can move or flow easily. It can be used to describe both liquids and gases that are not rigid and can change shape or be poured. Full definition
For the purposes of this publication, drawings are works on paper that do not contain a collage element and that are executed exclusively in one or more dry mediums (such as graphite, charcoal, or crayon), and / or in fluid mediums other than paint (such as ink, watercolor, or gouache, which is an opaque form of watercolor). (dedalusfoundation.org)
She has gradually loosened her paint surface into a more fluid medium with open brushworks while reopening an extended exploration of her emblematic star imagery. (arthaps.com)
Any painting that has gestural strokes I suppose could be called «expressionist» but it is not what the work is about, and oil paint is what it is, a very fluid medium that lends itself well to many differing applications no matter what you are painting. (artillerymag.com)