The primacy effect is found when the results of a free recall task are plotted in the form of a serial position curve. Generally, this curve is U-shaped, and the primacy effect corresponds to the tail of the U on the left. This tail indicates that words presented at the start of a list of to-be-remembered items are better remembered than words presented in the middle of this list. It is called the primacy effect because these items were the ones presented first to the subject in the memory experiment.
The primacy effect appears to be the result of subjects recalling items directly from a semantic memory. This is because the primacy effect can be sharply attenuated by performing manipulations that adversely affect this system -- such as using fast presentation of items (which does not permit much elaborative rehearsal to transfer memories from short-term to long-term stores), or by using list items that have similar meanings (and thereby producing semantic confusions).
The primacy effect was important to cognitive science because it provided empirical evidence for the decomposition of memory into an organized set of subsystems, which is required by functional analysis.