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Is there such a thing as “stock insurance”?
First off, the jargon you are looking for is a hedge. A hedge is "an investment position intended to offset potential losses/gains that may be incurred by a companion investment" (http://en.wikipedia.org/wiki/Hedge_(finance)) The other answers which point out that put options are frequently used as a hedge are correct. However there are other hedging instruments used by financial professionals to mitigate risk.  For example, suppose you would really prefer that Foo Corporation not go bankrupt -- perhaps because they own you money (because you're a bondholder) or perhaps because you own them (because you're a stockholder), or maybe you have some other reason for wanting Foo Corp to do well. To mitigate the risk of loss due to bankruptcy of Foo Corp you can buy a Credit Default Swap (http://en.wikipedia.org/wiki/Credit_default_swap). A CDS is essentially a bet that pays off if Foo Corp goes bankrupt, just as insurance on your house is a bet that pays off if your house burns down.   Finally, don't ever forget that all insurance is not just a bet that the bad thing you're insuring against is going to happen, it is also a bet that the insurer is going to pay you if that happens.  If the insurer goes bankrupt at the same time as the thing you are insuring goes bad, you're potentially in big trouble.