The cognitive area, skill, or trait measured.
Discrimination is highly related to reliability.
The set of all possible items which measure a construct. The domain is usually hypothetical. The particular questions or items on an assessment instruments are only a few of the possible items one could write to measure the desired construct. If we tested students again, with similar but different items, we would want the scores from the two tests to be correlated, or reliable. This can be estimated from the correlations of items within the instrument.