A supervisor, or also known as foreman, boss, overseer, facilitator, monitor, area coordinator, or sometimes gaffer, is the job title of a low level management position that is primarily based on authority over a worker or charge of a workplace. A supervisor can also be one of the most senior in the staff at the place of work, such as a Professor who oversees a PhD dissertation. Supervision, on the other hand, can be performed by people without this formal title, for example by parents. The term supervisor itself can be used to refer to any personnel who have this task as part of their job description.

An employee is a supervisor if he/she has the power and authority to do the following actions (according to the Ontario Ministry of Labor):

1. Give instructions and/or orders to subordinates.

2. Be held responsible for the work and actions of other employees.

If an employee cannot do the above, legally, he or she is probably not a supervisor, but in some other category, such as a work group leader or lead hand.

More Info: en.wikipedia.org