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Fixed air, or fixible air, is an ancient term for carbon dioxide
Joseph Priestley credited Joseph Black for discovering and coining "fixed air", which was thought to exist in a fixed state in alkaline salts, chalk, and other calcareous substances. Black considered substances containing fixed air to be "mild", and upon expulsion of the gas by heating the resulting state is "caustic" by corroding or burning plants and animals (e.g.  released by chalk upon decomposition to calcium oxide). In other words, the fixed air (also known as fixible air) was thought to be fixated within a corrosive molecule.
Priestley likewise credited the discovery of fixed air to contributions from several scientists including: David Macbride, John Pringle, William Brownrigg (regarded carbonated water to have an acidulous taste), Stephen Hales, and many others.
Henry Cavendish provided a definition: "By fixed air, I mean that particular species of factitious air, which is separated from alkaline substances by solution in acids or by calcination". Cavendish essentially defined potassium oxide or calcium oxide as a base, which can contain a fixated air within its composition, setting the stage for the historical definition of carbonate.