Alimony is an allowance for support made under court order to a divorced person by the former spouse, usually the chief provider during the marriage. Alimony may also be granted without a divorce, as between legally separated persons. Legally, نفقة المتعة could be considered a form of alimony, however, the term lacks some of the sense components of the word that are particular to the Islamic context. If your text is purely legal, I would use alimony, as non-Islamic societies don't have any other equivalent to نفقة المتعة, however, if you want to maintain the Islamic meanings of the term, I would use conciliatory settlement/payment/support and add a footnote.

The following is taken from a document that talks about family issues in Islam:
Islamic Shari’ah encourages the man to give his wife a material gift, known as al-Mut`ah (the conciliatory gift), when he divorces her. The value of the gift should be in proportion to the husband’s financial abilities and the duration of the marriage. The purpose of the gift is to console her and to lessen the harm that she endures as a result of the divorce.

Thus the main differences between alimony and al-Muta'ah are:
1. Alimony could be paid by the woman to the man if her financial situation is better than his where as al-muta'ah is solely from the man to the woman
2. Alimony doesn't address the aspect of trying to provide emotional solace to the woman after the divorce.


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Note added at 12 hrs (2009-02-02 11:52:04 GMT)
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also, conciliatory maintenance