My introductory text book on thermodynamics has just one ambiguous mention of something like "Isochoric expansion of an ideal gas " . But does it really make sense ? Or is it just a blunder ?
In his lectures on thermodynamics, Fermi defines an isochoric transformation as one in which the system doesn't do any external work. In this sense, the free expansion of a perfect gas (AKA Joule expansion) is an isochoric process.
However, I must say that that's the only place where I've seen an "isochoric process" defined in this way, the common definition is the one given by annav, so it might as well be just a blunder.
An isochoric process, also called a constant-volume process, an isovolumetric process, or an isometric process, is a thermodynamic process during which the volume of the closed system undergoing such a process remains constant. An isochoric process is exemplified by the heating or the cooling of the contents of a sealed, inelastic container: The thermodynamic process is the addition or removal of heat; the isolation of the contents of the container establishes the closed system; and the inability of the container to deform imposes the constant-volume condition
You should give more of the context where you found the quote. It might mean a "uniform expansion of volume". I found it here "expansion and compression of gas" so it might be describing an instant in an expansion.