(1) The representation of a set of N points (pairs of numbers) by a smooth function, of a single variable, containing fewer than N defining constants. This is a special case of smoothing and curve fitting. (2) Representation of a set of N points in space by a smooth surface in that space, the surface being specified by too few constants to allow it to pass through all the points. A fit is also called a smoothing or an adjustment; the process is called fitting, curve fitting or adjustment. (3) An expression of how well a curve or surface represents a given set of numbers i.e., a word or number used to give an idea of how far the given points lie from the curve or surface representing them. E.g.., to say that a fit is good is to imply that the points lie acceptably close to their geometric representation; to say that the fit is poor is to imply the opposite. The root mean square value of the distances is frequently used to indicate fit.