Null distribution, defined as the asymptotic distributions of null quantile-transformed test statistics  is in the procedure of hypothesis testing
The null distribution is defined as the asymptotic distributions of null quantile-transformed test statistics, based on marginal null distribution. In statistical hypothesis testing, the null distribution is the probability distribution of the test statistic when the null hypothesis is true. In the procedure of hypothesis testing, one needs to form the joint distribution of test statistics to conduct the test and control type I errors. However, the true distribution is often unknown and a proper null distribution ought to be used to represent the data. 
