The
duration during which a
camera's
shutter is open and the
film plane is
exposed to
light. These speeds are rated in
terms of 'stops', each stop being being a
duration twice as long as the previously
slower one... Thus: 1 second, 1/2 of a second, 1/4th, 1/8th, 1/15th, 1/30th...etc...
Certain minimum shutter speeds are necessary in order to stop certain kinds of action from appearing as movement in a photo. For example: