If @MaciejMaciej's and @GillesGilles's answer's don't solve your problem, and you don't recognize the process (and asking what it is with your distro doesn't turn up answers ). Check for Rootkit's and any other signs that you've been owned. A rootkit is more than capable of preventing you from killing the process. In fact many are capable of preventing you from seeing them. But if they forget to modify 1 small program they might be spotted ( e.g. they modified top, but not htop ). Most likely this is not the case but better safe than sorry.
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Gilles 'SO- stop being evil'
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