Noun

(en noun)

Something that constrains.

(mathematics) A condition that a solution to an optimization problem must satisfy.

A restriction.

Noun

(en noun)

A confrontation; a dare.

# An instigation or antagonization intended to convince a person to perform an action they otherwise would not.

#*{{quote-magazine, date=2013-11-30, volume=409, issue=8864, magazine=(The Economist), author=Paul Davis

, title= Letters: Say it as simply as possible , passage=Congratulations on managing to use the phrase “preponderant criterion” in a chart (“ On your marks”, November 9th). Was this the work of a kakorrhaphiophobic journalist set a challenge by his colleagues, or simply an example of glossolalia?}}

# A bid to overcome something.

#* {{quote-news, year=2012, date=May 5, author=Phil McNulty, work=BBC Sport

, title= Chelsea 2-1 Liverpool , passage=For Liverpool, their season will now be regarded as a relative disappointment after failure to add the FA Cup to the Carling Cup and not mounting a challenge to reach the Champions League places.}}

# (sports) An attempt to take possession; a tackle

#* {{quote-news, year=2011, date=October 1, author=Saj Chowdhury, work=BBC Sport

, title= Wolverhampton 1-2 Newcastle , passage=Argentine midfielder Jonas Gutierrez added a superb second when he surged past four challenges to fire in low.}}

# A summons to fight a duel; also, the letter or message conveying the summons.

# The act of a sentry in halting a person and demanding the countersign, or (by extension) the action of a computer system demanding a password, etc.

A difficult task, especially one that the person making the attempt finds more enjoyable because of that difficulty.

(label) A procedure or action.

# (legal, rare) A judge's interest in the result of the case for which he or she should not be allowed to sit the case, e.g. a conflict of interest.

# The act of appealing a ruling or decision of a court of administrative agency.

# The act of seeking to remove a judge, arbitrator or other judicial or semi-judicial figure for reasons of alleged bias or incapacity.

# (label) An exception to a person as not legally qualified to vote. The challenge must be made when the ballot is offered.

(label) The opening and crying of hounds at first finding the scent of their game.