Envelope

Envelope, in Geometry, means the line or surface formed by small portions of a series of curves or curved surfaces, such that if a point travels along the line or 'surface, it will at any instant be travelling tangentially to one of the series. If a great number of equal circles be drawn all passing through one point, their envelope will be a circle of double the linear dimensions and with its centre at the given point. Or if a great number of straight lines be drawn in a plane, all at the same distance from a given-point in it, they will envelope a circle with that point as centre.