A piece of software or a script that does Bad Things to data. For example delete or even worse alter data. The authors of such code are usually Bad Guys. This sort of code flourishes on the internet where there are many users lacking security knowledge.
This may also include code added to an open-source project intended to harm. This is the great fear that closed-source developers would have us worry about while most OSS advocates point out peer-review prevents such problems.
Examples for malicious code: