ImplicitRegion

ImplicitRegion[cond, {x1, ..., xn}] represents a region that satisfies the conditions cond.

Examples

Unit disk:

reg = ImplicitRegion[x^2 + y^2 <= 1, {x, y}];
RegionPlot[reg]

3D sphere:

ImplicitRegion[x^2 + y^2 + z^2 <= 1, {x, y, z}]

Please visit the official Wolfram Language Reference for more details.