A choice. Choosing not to do something that you want to do. Self-control.
See Sexual Abstinence
"I guess I should abstain from shopping this weekend; my credit card is maxed!"
"Yeah, I've had sex before, but I'm choosing abstinence now. I don't want herpes on my balls."
"I drank a lot in high school, but got tired of it. I guess you could say I'm "abstaining"--whatever--I don't have to drink to have a good time."