When is a supervisor not a supervisor? For purposes of Title VII, the U.S. Supreme Court has answered that question in a 5-4 vote and has imposed a more narrow definition. See the decision in Vance v. Ball State Univ., No. 11-556 (6/24/13) here.
The Court found that a person is a "supervisor" only if that person is empowered by the employer to take tangible employment action regarding an employee, that is, to affect a "significant change" is an employment status, such as hiring, firing, failing to promote, reassignment with significantly different responsibilities, or a decision causing a significant change in benefits. Id., at 15. The Court rejected the EEOC definition of supervisor: 1) someone authorized to take action or recommend action affecting an employee, including hiring, firing, transfers, etc., and 2) a person who is authorized to direct the employee's daily work activities. The Court found the more expansive definition to be based on "colloquial" usage of the term supervisor.
The Supreme Court relied on the decisions in Burlington Industries, Inc. v. Ellerth, 524 U.S. 742 (1998) and Faragher v. Boca Raton, 524 U.S. 775 (1998), seminal decisions regarding supervisor based harassment. As the dissent notes, the more expansive definition is also more flexible, recognizing the wide variety of employment situations. Employers are creative. They need to be. They are dealing with everyday life with the full range of the human experience. For example, the miscreant lifeguard supervisor in Faragher could impose toilet duty on female lifeguards who resisted his advances, but he could not take tangible employment actions against them. Yet, that same male senior lifeguard was enabled by his perceived authority in harassing the female lifeguards. Under the Vance decision, that male lifeguard would not be a "supervisor." Any legal action against him by an oppressed female lifeguard would fail.
So, yes, colloquial definitions should apply whenever the court is dealing with what, after all are "colloquial" situations. This decision in Vance undermines its prior holding in Faragher. Worse, the newest decision simply does not reflect the reality of a more modern, flexible workplace.