A method of representing a 3D object on a 2D plane used in technical and engineering drawings. Unlike in perspective drawings or real life, parallel lines do not converge to a vanishing point and objects appear to be the same size regardless of distance.
Note that this tag refers to what is technically called axonometric projection, of which isometric projection is a special case. Consequently, many images tagged as isometric on Danbooru may actually be dimetric or trimetric.