Common meaning:

The difference in pay between what a woman receives and what a man gets for doing exactly the same job.

Hidden meaning of the Equity religion: 

Females will be paid less because they are oppressed, so we just have to find the evidence of this occurring.

Discussion:

The Equityists think that men have been oppressing women for a long time and that one of the ways this shows up is the fact that women are paid less for doing the same job.

Because believers of Equity already know that men oppress women, any evidence that indicates the gender wage gap is not real or is caused by other factors, must be false. They know by faith that the gap is real and is completely caused by sexism and nothing else. The only real question is, How big is the gap?

Back in reality, we find that although a small fraction of the pay gap might be the result of sexism, the vast majority of the gap can easily be explained by other factors, such as the fact that women tend to work fewer years because they have babies.

If you assume that the only possible reason for differences in pay is sexism, you will find what you are looking for, even if you have to make up the numbers. 

Heaven help us all.