Subitizing is a phenomenon in which the number of items in a set of objects (the cardinality of the set) can be effortlessly and rapidly detected if the set has four or fewer items (Jensen, Reese, & Reese, 1950; Kaufman, Lord, Reese, & Volkmann, 1949).  Larger sets cannot be subitized; a much slower process is required to serially count the elements of larger sets.  Subitizing necessarily requires that the items to be counted are individuated from one another.  Trick and Pylyshyn (1993, 1994) have used subitizing to support the hypothesis that this type of automatic counting could be accomplished by primitive mechanisms of attentional indexing; elements are preattentively individuated by being indexed, and counting simply requires accessing the number of indices that have been allocated.  When conditions are manipulated to prevent preattentive individuation or indexing to occur, subitizing fails!

References:

1. Jensen, E. M., Reese, E. P., & Reese, T. W. (1950). The subitizing and counting of visually presented fields of dots. Journal of Psychology, 30(2), 363-392.
2. Kaufman, E. L., Lord, M. W., Reese, T. W., & Volkmann, J. (1949). The discrimination of visual number. American Journal of Psychology, 62(4), 498-525.
3. Trick, L. M., & Pylyshyn, Z. W. (1993). What enumeration studies can show us about spatial attention: Evidence for limitied capacity preattentive processing. Journal of Experimental Psychology: Human Perception and Performance, 19(2), 331-351.
4. Trick, L. M., & Pylyshyn, Z. W. (1994). Why are small and large number enumerated differently: A limited-capacity preattentive stage in vision. Psychological Review, 101(1), 80-102.

(Added March 2011)