What is a tithe?

Some of the above debate has reminded me of a discussion around here, locally- involving tithing. Specifically, how does one determine the 10% to donate? Is it 10% of gross? 10% of AGI? 10% of net? 10% of what you have left after would have paid your "regular bills" (eg house, utilities, food and health insurance, but before savings, vacation, hair, makeup.....)) - aka "discretionary income"?

I was surprised by several people's answers.