a character of special significance in an organism. Such characters are used in a system for determining the identity of an organism on the basis of the presence or absence of a succession of characters. Usually such keys are DICHOTOMOUS (2); that is to say, at each step there is a choice between two possibilities. Keys, however, may have more than two characters at each step and there is a gradual elimination of absent characters until identification is made.
In 1993, the federal government began an aggressive campaign of building security barriers in key areas along the 2,100-mile border with Mexico and adding hundreds of patrol agents in an effort to stem the tide of immigration.