What is Abstention?

Legal Definition
Abstention is a term in election procedure for when a participant in a vote either does not go to vote (on election day) or, in parliamentary procedure, is present during the vote, but does not cast a ballot. Abstention must be contrasted with "blank vote", in which a voter casts a ballot willfully made invalid by marking it wrongly or by not marking anything at all. A "blank (or white) voter" has voted, although their vote may be considered a spoilt vote, depending on each legislation, while an abstaining voter hasn't voted. Both forms (abstention and blank vote) may or may not, depending on the circumstances, be considered to be a protest vote (also known as a "blank vote" or "white vote").

An abstention may be used to indicate the voting individual's ambivalence about the measure, or mild disapproval that does not rise to the level of active opposition. Abstention can also be used when someone has a certain position about an issue, but since the popular sentiment supports the opposite, it might not be politically expedient to vote according to his or her conscience. A person may also abstain when they do not feel adequately informed about the issue at hand, or has not participated in relevant discussion. In parliamentary procedure, a member may be required to abstain in the case of a real or perceived conflict of interest.

Abstentions do not count in tallying the vote negatively or positively; when members abstain, they are in effect attending only to contribute to a quorum. White votes, however, may be counted in the total of votes, depending on the legislation.
-- Wikipedia
Legal Definition
A federal court's decision not to exercise jurisdiction over a case. The usual goal of abstention is the avoidance of needless conflict with a state court. See Federalism.
Abstention: an overview
Abstention is a doctrine under which federal courts may choose not to hear a case, even if all the formal jurisdiction requirements are met. There are several established instances in which federal courts will generally abstain. First, federal courts will abstain rather than issue an injunction against a state court, in either a civil or criminal matter. See Klein v Burke Constr. Co, 260 US 266 (1922); Railroad Comm'n of Texas v. Pullman Co., 312 US 496 (1941).
Legal Definition
In French law. Keeping an heir from possession; also tacit renunciation of a succession by an heir. Merl. Repert.
-- Black's Law Dictionary
Legal Definition
French law. This is the tacit renunciation by an heir of a succession Merl. Rep. h.t.
-- Bouviers Law Dictionary