In hypothesis testing, the specific value of the parameter or parameter function under the null hypothesis.
1. In a one sample z-test of a population mean, you may test whether the mileage of your car is equal to 35 mpg, or different from this value. Here 35 mpg is the test value. The null hypothesis is H0: μ = 35 mpg.
2. In a two sample t-test, you may want to test whether or not the difference in the average cyle times of two products is 3 hours, or more than 3 hours. Here the test value (also called 'Difference of interest') is 3 hours.
H0: μ1 - μ2 = 3 hours.