The term "nomological" is derived from Greek and means "lawful". Briefly, the nomological network was Cronbach and Meehl's view of construct validity (Cronbach, L. J., & Meehl, P. E. (1955). Construct validity in psychological tests. Psychological bulletin, 52(4), 281). That is, in order to provide evidence that your proposed measure has construct validity, Cronbach and Meehl argued that you had to develop a nomological network for your measure. This network would include the theoretical framework for what you are trying to measure, an empirical framework for how you are going to measure it, and specification of the linkages among and between these two frameworks.
Perhaps some context would affect how I answer this question.
The “nomological net” refers to the theory and theoretical constructs within which a construct is embedded. ”Nomological validity” would mean something along the lines of ‘is the construct being measured related to measures of constructs that are theoretically related to it in ways predicted by the theory that the construct is derived from?’
The term "nomological" is derived from Greek and means "lawful". Briefly, the nomological network was Cronbach and Meehl's view of construct validity (Cronbach, L. J., & Meehl, P. E. (1955). Construct validity in psychological tests. Psychological bulletin, 52(4), 281). That is, in order to provide evidence that your proposed measure has construct validity, Cronbach and Meehl argued that you had to develop a nomological network for your measure. This network would include the theoretical framework for what you are trying to measure, an empirical framework for how you are going to measure it, and specification of the linkages among and between these two frameworks.