Any variable that you're not investigating that can potentially affect the outcomes of your research study, which can lead to inaccurate conclusions and relation between IV and DV
there are three types
Participant variables
Any characteristic or aspect of a PPs background that could affect study results, even though it's not the focus of the experiment
Situational variables
Factors in the environment that could affect the PPs performance
Experimenter variables
When the researcher unintentionally influences how PPs behave
