curvilinear
adj. describing an association between variables that does not consistently follow an increasing or decreasing pattern but rather changes direction after a certain point (i.e., it involves a curve in the set of data points). For example, the relationship between anxiety and achievement often has a curvilinear pattern of increasing achievement with increasing anxiety (i.e., motivation to study) up to a certain point when there is so much anxiety that achievement tends to decrease. Thus, individuals who are not at all anxious and those who are extremely anxious would both be expected to have poor performance, whereas moderately anxious individuals would be expected to have reasonably high performance. See also nonlinear.