This has been asked a number of times before, and you might want to do a keyword search on the term.

In the US, "severability" (or severability clause) seems to be most prevalent.

Schäfer on the subject, including an example:
salvatorische Klausel f
(Re) separability
– severability
– saving . . . clause
(ie, die Klausel soll der Regelung des § 139 BGB entgegenwirken; sie verhindert jedoch keine Lücken im Vertragswerk; häufige Formulierung:
,,Sollte e–e dieses Vertrages unwirksam oder undurchführbar sein oder werden, so berührt dies die Wirksamkeit des Vertrages im übrigen nicht. Die Parteien verpflichten sich vielmehr, in einem derartigen Fall eine wirksame oder durchführbare Bestimmung an die Stelle der unwirksamen oder undurchführbaren zu setzen, die dem Geist und Zweck der zu ersetzenden Bestimmung so weit wie möglich entspricht."
,,Should any individual provision or any part of any provision be or become void, illegal or unenforceable, the validity of the remaining provisions hereof shall in no way be affected. In such case the void and/or illegal and/or unenforceable provision or provisions shall be replaced by relative provisions coming as close as possible to the sense and spirit and purpose of this Agreement.")

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Note added at 2002-04-21 17:34:18 (GMT)
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The contract itself generally only uses the term \"Severability\" (without adding the word clause).
Just this week I had to review a lengthy lease option contract for our homeowner\'s association, after it had been through our attorney\'s and the other party\'s attorney\'s hands, and \"Severability\" was, of course, also mentioned - and, as noted above, without the addition of the word \"clause\".

This is a clause saying roughly that if in any of the other clauses a mistake has been made this does not voud the whole contract.

This is a popular usage. Where part of a provision is contrary to law, the valid part is "saved" by the protective clause.