n. 1. also called linear eccentricity. a parameter, often denoted e, that identifies the shape of a conic section as a locus of points such that the ratio between the distance of any such point from a given fixed point (the focus) and its distance from a given fixed line (the directrix) equals this constant. Clearly this constant is independent of the position, orientation and size of the curve, and so identifies a family of similarly shaped curves. 2. also called numerical eccentricity. the ratio of the linear eccentricity to half the length of the major axis of a conic, often denoted ɛ, that is constant for a family of similar curves. If the equation of the curve is given in vertex form as with 2p the length of the latus rectum, then, as shown below, if ɛ = 0, the curve is a circle; if ɛ < 1, the curve is an ellipse; if ɛ = 1, it is a parabola; and if ɛ > 1, it is a hyperbola. Compare ellipticity. Graphs of y² = 2x - (1 - ɛ²) x² for ɛ shown.