How long the camera shutter remains open to let light onto the film
Shutter speed denotes the time that the shutter remains open when taking a picture. It is measured in fractions of a second (so you can imagine that each number has a 1/ in front of it): the bigger the number, the faster the speed. Just as with aperture stops, shutter speeds are set as stops on a scale.
Typical shutter speed scale:
1 1/2 1/4 1/8 1/15 1/30 1/60 1/125 1/250 1/500
Each of the numbers in this sequence (each stop) represents a halving to the right, or doubling to the left of the amount of light from its immediate neighbour.
If you want to avoid motion blur and achieve sharp images, assuming a normal camera and a steady hand, you should use a minimum shutter speed equal to 1 over the focal length of the lens i.e. a 28 mm lens can be held steady at 1/28 (1/30). If your shutter speed is slower than the reciprocal of the focal length of your lens, you should use a tripod to achieve a sharp image.
As a general rule you can’t achieve sharp shots at speeds slower than 1/60, but with rangefinder cameras (see Rangefinder Vs. SLR) because there is no vibration from mirror slap people say they can go down to 1/30 although this is below the reciprocal of a 45mm lens. Legend has it that Cartier Bresson could shoot hand held at 1/4, but as a rule stick to 1/60 and you’ll avoid unwanted motion blur.