APA Dictionary of Psychology

a method for assigning study participants to experimental conditions in which individuals are arbitrarily divided into subsets or blocks and then some random process is used to place individuals from those blocks into the different conditions. For example, a researcher might divide participants into blocks of 10 and then randomly assign half of the people in each to the control group and half to the experimental group. Block randomization is distinct from blocking in that the block does not have any significance other than as an assignment unit.