A contaminant is something that shouldn't be present in a substance and that has corrupted the purity or integrity of the substance.
For example, water can be contaminated with chemicals or lead making it dangerous to drink. In research a contaminant is something that has entered a test substance that isn't supposed to be there.
For example, researchers must take great care in DNA testing not to contaminate the sample with their own DNA. This would make the results invalid and inconclusive.
Another example would be testing the sodium levels in a mouse. A possible contaminant could be sweat from the researcher which would skew the sodium levels making them appear higher than they actually are.