Geometrically, an elevation is a horizontal orthographic projection a building on to a vertical plane, the vertical plane normally being parallel to one side of the building. Architects also use the word elevation as a synonym for façade, so the north elevation is literally the north-facing wall of the building.

house elevations are two dimensional pictures of the façade of a house or interior features that have faces projecting from a flat, solid background, like cabinetry, fireplaces, niches and elaborate wall designs

However, for the vast majority of houses it will be straightforward to identify which elevation is the 'principal elevation'. Once you have decided which is the principal elevation, you will then know which are the rear and side elevations.