AskDefine | Define speed

Dictionary Definition



1 distance travelled per unit time [syn: velocity]
2 a rate (usually rapid) at which something happens; "the project advanced with gratifying speed" [syn: swiftness, fastness]
3 changing location rapidly [syn: speeding, hurrying]
4 the ratio of the focal length to the diameter of a (camera) lens system [syn: focal ratio, f number, stop number]
5 a central nervous system stimulant that increases energy and decreases appetite; used to treat narcolepsy and some forms of depression [syn: amphetamine, pep pill, upper]


1 step on it; "He rushed down the hall to receive his guests"; "The cars raced down the street" [syn: rush, hotfoot, hasten, hie, race, pelt along, rush along, cannonball along, bucket along, belt along] [ant: linger]
2 move faster; "The car accelerated" [syn: accelerate, speed up, quicken] [ant: decelerate]
3 travel at an excessive or illegal velocity; "I got a ticket for speeding"
4 move very fast; "The runner zipped past us at breakneck speed" [syn: travel rapidly, hurry, zip]
5 cause to move faster; "He accelerated the car" [syn: accelerate, speed up] [ant: decelerate] [also: sped]

User Contributed Dictionary



spēd, from Germanic *spodiz. Cognate with Dutch spoed.


  • /spiːd/
  • Rhymes with: -iːd


  1. the state of moving quickly or the capacity for rapid motion; rapidity
  2. the rate of motion or action, specifically / the magnitude of the velocity; the rate distance is traversed in a given time
  3. the sensitivity to light of film, plates.
  4. any amphetamine drug used as a stimulant, especially illegally, especially methamphetamine
  5. luck, success, prosperity



rate of motion
photographic speed
slang: methamphetamine
luck, success

Usage notes



  1. In the context of "archaic|intransitive": To prosper, be lucky.
  2. In the context of "archaic|transitive": To help someone, to give them fortune.
    God speed, until we meet again.
  3. To go (too) fast.
    The Ferrari was speeding along the road.
  4. To exceed the speed limit.
    My mum got yet another fine for speeding.

Usage notes

The Cambridge Guide to English Usage indicates that sped is for objects in motion (the race car sped) while speeded is used for activities or processes, but notes that the British English convention does not hold in American English.


to go fast
to exceed speed limit
translations to be checked


Extensive Definition

Speed is the rate of motion, or equivalently the rate of change in position, often expressed as distance d traveled per unit of time t.
Speed is a scalar quantity with dimensions distance/time; the equivalent vector quantity to speed is known as velocity. Speed is measured in the same physical units of measurement as velocity, but does not contain the element of direction that velocity has. Speed is thus the magnitude component of velocity.
In mathematical notation, it is simply:
v = \left|\frac \right|.
Note that "v" is the variable for speed.
Objects that move horizontally as well as vertically (such as aircraft) distinguish forward speed and climbing speed.


Units of speed include:
Mach 1 ≈ 343 ms-1 ≈ 1235 km/h ≈ 768 mph in dry air at sea-level pressure and 293 kelvin (See Speed of sound for more detail.)
c = 299,792,458 ms-1
  • Other important conversions
1 m/s = 3.6 km/h
1 mph = 1.609 km/h
1 knot = 1.852 km/h = 0.514 ms-1
Vehicles often have a speedometer to measure the speed they are going.

Average speed

Speed as a physical property represents primarily instantaneous speed. In real life we often use average speed (denoted |\tilde|), which is rate of total distance (or length) and time interval. For example, if you go 60 miles in 2 hours, your average speed during that time is 60/2 = 30 miles per hour, but your instantaneous speed may have varied.
In mathematical notation:
|\tilde| = \frac
Instantaneous speed defined as a function of time on interval [t_0, t_1] gives average speed:
|\tilde| = \frac
while instant speed defined as a function of distance (or length) on interval [l_0, l_1] gives average speed:
|\tilde| = \frac
It is often intuitively expected, but incorrect, that going half a distance with speed |v|_ and second half with speed |v|_, produces total average speed |\tilde| = \frac. The correct value is |\tilde| = \frac (Note that the first is a proper arithmetic mean while the second is a proper harmonic mean).
Average speed can be derived also from speed distribution function (either in time or on distance):
|v| \sim D_t\; \Rightarrow \; |\tilde| = \int |v| D_t(|v|) \, dv
|v| \sim D_l\; \Rightarrow \; |\tilde| = \frac

Examples of different speeds

Below are some examples of different speed (see also main article Orders of magnitude (speed)):
  • Speed of a common snail = 0.001 ms-1; 0.0036 km/h; 0.0023 mph.
  • A brisk walk = 1.667 ms-1; 6 km/h; 3.75 mph.
  • Olympic sprinters (average speed over 100 metres) = 10 ms-1; 36 km/h; 22.5 mph.
  • Speed limit on a French autoroute = 36.111 ms-1; 130 km/h; 80 mph.
  • Top cruising speed of a Boeing 747-8 = 290.947 ms-1; 1047.41 km/h; 650.83 mph; (officially Mach 0.85)
  • Official air speed record = 980.278 ms-1; 3,529 km/h; 2,188 mph.
  • Space shuttle on re-entry = 7,777.778 ms-1; 28,000 km/h; 17,500 mph.
  • the speed of sound in air (Mach 1) is about 340 ms-1, and 1500 ms-1 in water
  • Taipei 101 Observatory Elevator = 1010 m/min ; 16.667 ms-1 ; 60.6 km/h; 37.6 mph


speed in Afrikaans: Spoed
speed in Arabic: سرعة
speed in Catalan: Rapidesa
speed in Welsh: Buanedd
speed in Danish: Fart
speed in German: Geschwindigkeit
speed in Estonian: Kiirus
speed in Spanish: Rapidez
speed in Esperanto: Rapido
speed in French: Vitesse
speed in Korean: 속력
speed in Croatian: Brzina
speed in Ido: Rapideso
speed in Icelandic: Hraði
speed in Italian: Velocità media
speed in Hebrew: מהירות
speed in Georgian: სიჩქარე
speed in Latvian: Ātrums
speed in Lithuanian: Greitis
speed in Macedonian: Брзина
speed in Malay (macrolanguage): Laju
speed in Dutch: Snelheid
speed in Japanese: 速さ
speed in Norwegian: Fart
speed in Norwegian Nynorsk: Fart
speed in Polish: Szybkość
speed in Portuguese: Rapidez
speed in Romanian: Viteză
speed in Russian: Скорость
speed in Simple English: Speed
speed in Finnish: Vauhti
speed in Swedish: Fart
speed in Thai: อัตราเร็ว
speed in Ukrainian: Швидкість
speed in Chinese: 速率

Synonyms, Antonyms and Related Words

Benzedrine, Benzedrine pill, C, Dexamyl, Dexamyl pill, Dexedrine, Dexedrine pill, Methedrine, abruptness, accelerate, advance, advantage, aid, air speed, alacrity, amphetamine, amphetamine sulfate, assist, ball the jack, barrel, bat, bolt, boom, boost, bowl along, breeze, breeze along, briskness, brush, bucket, bundle, burn up, bustle, career, celerity, charge, chase, cheer on, clear the way, clip, cocaine, coke, conduce to, contribute to, crowd, crystal, cut along, dart, dextroamphetamine sulfate, dispatch, drive on, ease, egg on, encourage, expedite, expedition, expeditiousness, explain, facilitate, fastness, favor, feverishness, fleet, fleetness, flight, flit, flurry, fly, fly low, foot, football, forward, furiousness, further, gait, go fast, goad on, grease, grease the ways, grease the wheels, ground speed, haste, hasten, hasten on, hastiness, headway, heart, help, help along, hickory, hie on, highball, hound on, hurriedness, hurry, hurry along, hurry on, hurry up, hustle, hustle up, impetuosity, impetuousness, impulsiveness, instantaneousness, jolly bean, knots, legerity, lend wings to, lightning speed, loose, lubricate, make clear, make for, make haste, make knots, make tracks, make way for, methamphetamine hydrochloride, miles per hour, move, nip, oil, open the way, open up, outstrip the wind, pace, pave the way, pep pill, pour it on, precipitance, precipitancy, precipitate, precipitateness, precipitation, precipitousness, prepare the way, press, promote, promptitude, promptness, purple heart, push, push forward, push on, push through, put forward, quicken, quickness, race, railroad through, rapidity, rashness, remove friction, rip, root on, round pace, rpm, run, run interference for, rush, rush along, rustle, scamper, scorch, scramble, scurry, set forward, shake up, shock wave, shoot, simplify, sizzle, skedaddle, skim, smooth, smooth the way, snappiness, snow, soap the ways, sonic barrier, sonic boom, sound barrier, speed along, speed of sound, speed on, speed up, speediness, sprint, spur, spur on, stampede, step on it, step up, stimulant, storm along, streak, suddenness, sweep, swift rate, swiftness, tear, tear along, thunder along, timeliness, unbar, unblock, unclog, unjam, upper, urge, urge on, velocity, whip, whip along, whip on, whisk, whiz, zing, zip, zoom
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