A távollét ebben az esetben szerintem LEAVE. A tartós long-term (van short-term is). Semmiképp nem permanent vagy indefinite, mert ezeket inkább bevándorlási ügyekben használják. Oktatásban pedig leave of absence a távollét, ha jól tudom.

"Q:
Does Title VII require covered employers to give employees leave to care for an ill child or family member?
A:
Title VII in itself does not require employers to give employees leave to care for an ill child or family member. However, Title VII prohibits covered employers from discriminating on the basis of race, color, religion, sex, or national origin when they administer family leave."
http://www.eeoc.gov/policy/docs/fmlaada.html

"LONG-TERM FAMILY-RELATED LEAVE
Upon request from an eligible ASE, and subject to the provisions of this article, the University will grant an ASE’s reasonable request for long-term leave of absence during the academic year. Whenever possible leave should be requested at least thirty (30) days in advance of the begin date of the leave. The leaves described below may be combined for a maximum of four (4) weeks of leave during the academic year. The leave cannot be continued beyond the end date of the ASE’s appointment."
http://www.ucop.edu/atyourservice/employees/policies_employe...

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Note added at 36 mins (2011-04-09 09:12:53 GMT)
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Valóban, először én is így írtam "to be on long-term leave", majd valami miatt átjavítottam igés alakra... Köszönöm a visszajelzést mindkét Jánosnak! :)