The HR Dictionary
At-will employment is an employer's ability to dismiss an employee for any reason (that is, without having to establish "just cause" for termination), and without warning, as long as the reason is not illegal (e.g. firing because of the employee's race, religion or sexuality). Courts reject any claims for damages resulting from a dismissal where an employee is recognized as having been hired ‘at will.’ The rule's supporters argue that an employee may have a similar right to quit their work without cause or prior notice. Those who believe that the job relationship is characterized by an imbalance of bargaining power perceive the practice as being unjust.
Many jurisdictions changed the norm during the course of the 20th century by adding an increasing number of exceptions or by completely altering the employment contract's default expectations. The usual requirement for termination is that the employer must have "just cause," which is common in occupations in the public sector and places of employment where a trade union is recognized for the purposes of collective bargaining. However, most states follow the basic premise that an employer and employee may agree to the dismissal protection they like, subject to statutory rights (especially the restrictions against discrimination under the Civil Rights Act).