In hypothesis testing, the test procedure partitions all the possible sample outcomes into two subsets (on the basis of whether the observed value of the test statistic is smaller than a threshold value or not). The subset that is considered to be consistent with the null hypothesis is called the "acceptance region"; another subset is called the "rejection region" (or "critical region").
If the sample outcome falls into the acceptance region, then the null hypothesis is accepted. If the sample outcome falls into the rejection region, then the null hypothesis is rejected (i.e. the alternative hypothesis is accepted).