Physics The rate or a measure of the rate of motion, especially: Distance traveled divided by the time of travel. The limit of this quotient as the time of travel becomes vanishingly small; the first derivative of distance with respect to time. The magnitude of a velocity. Swiftness of action: He wrote the first chapter with great speed. The act of moving rapidly: finished the race in a burst of speed. The state of being in rapid motion; rapidity: The river's speed made a rescue difficult. A transmission gear or set of gears in a motor vehicle: What speed is the car in now? A numerical expression of the sensitivity of a photographic film, plate, or paper to light. The capacity of a lens to accumulate light at an appropriate aperture. The length of time required or permitted for a camera shutter to open and admit light. Slang A stimulant drug, especially amphetamine or methamphetamine. Slang One that suits or appeals to a person's inclinations, skills, or character: Living in a large city is not my speed. Archaic Prosperity; luck. To go, move, or proceed quickly: sped to the rescue. To drive at a speed exceeding a legal limit: was speeding on the freeway. To pass quickly: The days sped by. The months have sped along. To move, work, or happen at a faster rate; accelerate: His pulse speeded up. Slang To be under the influence of a stimulant drug. Archaic To prove successful; prosper. To get along in a specified manner; fare. To cause to move or proceed quickly; hasten: no wind to speed the boat. To increase the speed or rate of; accelerate. Often used with up: speed up a car; sped up production. To further, promote, or expedite (a legal action, for example). Archaic To help to succeed or prosper; aid. Operating at maximum speed. Producing something or performing at an acceptable rate or level. Informal Fully informed; conversant: I'm not up to speed on these issues yet.