A central nervous system stimulant that increases energy and decreases appetite; used to treat narcolepsy and some forms of depression the ratio of the focal length to the diameter of a (camera) lens system changing location rapidly a rate (usually rapid) at which something happens; "the project advanced with gratifying speed" distance travelled per unit time cause to move faster; "He accelerated the car" move very fast; "The runner zipped past us at breakneck speed" travel at an excessive or illegal velocity; "I got a ticket for speeding" move faster; "The car accelerated" step on it; "He rus