n.
Function: noun
Date: 14th century
1 a : a summons that is often threatening, provocative, stimulating, or inciting specifically : a summons to a duel to answer an affront b : an invitation to compete in a sport
2 a : a calling to account or into question : PROTEST b : an exception taken to a juror before the juror is sworn c : a sentry's command to halt and prove identity d : a questioning of the right or validity of a vote or voter
3 : a stimulating task or problem <looking for new challenge s >
4 : the act or process of provoking or testing physiological activity by exposure to a specific substance especially : a test of immunity by exposure to an antigen