Question: If you have a steak medium to rare, such that it's still pink in the middle, is it kosher? At what point is meat cooked *enough*?

Answer: 
What Monica answered above, that it is 'meat juice'. To find out in much greater detail about this, see [here](http://www.diaspora.org.il/Jewish_Law/on-line/05-milica.htm):

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> 1. No matter what the source of the issur, the Ran asks: We clearly observe that blood continues to come out of meat even after the salting process is over. Indeed this blood is sometimes even redder than the blood that was originally expelled. Why do we permit this meat after the required time of salting, even though there blood still appears to be inside?
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> He gives two explanations :
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> A. The liquid which comes out of the meat after the period of salting is not blood but is called mohul (juice). The Gemora refers to it as chamra boser (the wine of the meat). In other words, although it may have the appearance of blood, it is not in reality blood that is ossur at all.
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> B. The issur of blood that comes out through the salting process is only forbidden Rabbinically, and the Rabbis limited their prohibition to blood that comes out of the meat during the period of salting . After that time, anything that comes out of the meat, although it may be blood, it is not within the Rabbinical enactment. Therefore it is blood of heter and not of issur.
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This, based on the gemara in Chullin 113b.
Most of the other answers seem to be working under the assumption that the OP is basing the question on the prohibition against consuming blood (although this motivation is not stated in the question). Taking the question at its face value, however, the OP just wants to know how well meat needs to be cooked in order to be Kosher. As YDK's answer suggests (albeit somewhat vaguely), cooking has nothing to do with the Kashering process. Yes, we could get technical and discuss roasting meat in lieu of salting as a means of blood-removal, etc., but if the question is taken at face value, it is based on a faulty premise, probably due to a misunderstanding, that meat needs to be cooked 'X' amount in order to be Kosher. It does not.