In null hypothesis statistical testing, the critical region (also known as the rejection region) is
A set of values for the test statistic for which the null hypothesis is rejected. i.e. if the observed test statistic is in the critical region then we reject the null hypothesis and accept the alternative hypothesis. 
For a Stats 101-like course, they might be illustrated as follow (omitting most of the details):
The question is: How do you call the complement of the critical region / rejection region (the interval where the distribution is not shaded)? Is there a specific term for this interval?
I have heard of the interval being referred to as "non-rejection region" and "acceptance region" (which IMO is misleading as it implies acceptance of the null), though I am looking for answers with reference(s) that have defined / discussed on such terminology.
Note: I have seen this question but the question deals with the Bayesian version.
 Newcastle University (2018) Critical Region and Confidence Interval. https://internal.ncl.ac.uk/ask/numeracy-maths-statistics/statistics/hypothesis-testing/critical-region-and-confidence-interval.html