monotonic
adj. denoting a variable that either increases or decreases as a second variable either increases or decreases, respectively: The relationship is not necessarily linear but there are no changes in direction. A monotonically increasing variable is one that rises consistently as a second variable increases, for example, level of performance in relation to amount of practice if this were observed to be the case. In contrast, depression would be a monotonically decreasing variable if its severity were found to fall consistently as a person’s level of perseveration declined.