Eso...

No he eschuchado una forma canónica de decirlo pero aquí van unas opciones que me parece son apropiadas.

Saludos y espero que sea de utilidad.

Diego.

Se trata de un contrato a voluntad, cuya continuidad depende del libre albedrío de las partes.


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Note added at 19 hrs (2009-12-21 19:42:16 GMT)
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I see that additional reference is needed.

Glossary
of Compensation
Terms
U.S. Department of Labor
Alexis M. Herman, Secretary
Bureau of Labor Statistics
Katharine G. Abraham, Commissioner
http://www.dol.gov/search/AdvSearch.aspx?search_term="at wil...

EMPLOYMENT-AT-WILL
The theory that employers have the power to hire and fire workers solely at their own discretion.

http://legal-dictionary.thefreedictionary.com/Employment at ...
“A common-law rule that an employment contract of indefinite duration can be terminated by either the employer or the employee at any time for any reason; also known as terminable at will.
Traditionally, U.S. employers have possessed the right to discharge their employees at will for any reason, be it good or bad. The "at-will" category encompasses all employees who are not protected by express employment contracts that state that they may be fired only for good cause. "Good cause" requirements are typically a part of collective bargaining agreements negotiated by employee unions; nonunion workers rarely have this form of protection. The at-will doctrine also does not apply to contracts for a specified term, such as an employment contract that contemplates the employee providing service for an expressly designated number of years.”
http://employeeissues.com/at_will_employment.htm
In other words, at will employment means that employees may generally quit their jobs at anytime and for any, no or even unfair reasons.
By the same token, employers may generally fire or layoff employees at anytime and for any, no or even unfair reasons. (That's the inference of the saying, "I serve at the pleasure of the board of directors.") However, employers are limited by the exceptions noted on the next page.
At will employment also means that employers may decline to hire job candidates for any, no or even unfair reasons, except for those noted on the next page.

http://www.businessdictionary.com/definition/employment-at-w...


Contract of employment that can be terminated either by the employer or the employee at any time and for any reason.


http://www.nolo.com/dictionary/at-will-employment-term.html

Nolo's Plain-English Law Dictionary > Definition of "at-will employment"

Nolo’s Plain-English Law Dictionary
Legal Dictionary Home
at-will employment
An employment arrangement in which the employee may quit at any time, and the employer may fire the employee for any reason that is not illegal. For example, an employer may fire an at-will employee for poor performance, to cut costs, or because the employer simply doesn't like the employee, but may not fire an at-will employee for discriminatory reasons, to retaliate against the employee for reporting harassment, or because the employee exercised a legal right.



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Note added at 22 hrs (2009-12-21 22:55:26 GMT)
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I would like to explain that my answer refers to contracts in the United States.

Other countries do not have this specific type of contract. In Colombia, some public employees are called “de libre nombramiento y remoción.”