1.n. an assessment carried out after the application of some intervention, treatment, or other condition to measure any changes that have occurred. Posttests often are used in research contexts, in conjunction with pretests, to isolate the effects of a variable of interest. For example, in a study examining whether a new therapy helps to alleviate depression, participants might receive the therapy and then complete a short symptom inventory, the results of which would be compared to those from an inventory taken prior to the treatment. 2.vb. to administer a posttest.