to restrain or hold oneself back from doing something, typically something enjoyable; to choose not to engage in something, usually an act of self-denial;
e.g. Her doctor advised her to abstain from eating dairy to see if it made her feel better.
(verb)
to decline to vote on an issue entirely, i.e. to refuse to vote either for or against, usually formally;
e.g. At least 40 people abstained, while only about 20 voted for the opposition.