Unimodal (Curve) of a frequency distribution consists of only a single mode, i.e., only one peak. [Also see, Bimodal Distribution].
Usage-Use have two different functions in the language tests. 'Use' is concerned with the communicative value of the test response, whereas 'usage' is concerned with the accuracy or acceptability of the language patterns used within the sentence. [Also see, M's - the Three M's].