The limit of this quotient as the time of travel becomes vanishingly small; the first derivative of distance with respect to time, The capacity of a lens to accumulate light at an appropriate aperture, Success; a successful course; prosperity in doing something; good fortune; luck_ used either absolutely or relatively_ as, to wish one good speed in an undertaking, intransitive verb_ To go, move, or proceed quickly, To drive at a speed exceeding a legal limit, To prove successful; prosper