Definitions for "Confounder"
Confounding bias is caused by the presence of an extraneous factor associated with both an exposure under study and a disease outcome. A commonly used method to adjust for a potential confounder is stratification in which the comparison between exposure and disease is done at specific levels of the potential confounder. When a study mentions that they controlled for a factor, they have tried to remove the effect of that variable. While confounding is a bias which an investigator wishes to eliminate, effect modification refers to a difference in the magnitude of an effect measure across levels of another variable. An example is throat cancer (disease) and high alcohol use (risk factor). An effect modifier is smoking, since the relative risk for alcohol has been reported to be greater at higher levels of smoking. More accurately this should be called risk-ratio modification since effect modification also depends on the risk scale for the outcome, for example risk ratio or risk difference.
If you don't know whether an effect is caused by the variable you are interested in (e.g. a drug or smoking) or by another variable (e.g. age or sex) then the other variable is called a confounder and it is said to cause confounding. A confounder is a variable other than the one being investigated which is asociated with both the exposure and the outcome. A confouder can cause bias in a study.
A condition or variable that may be a factor in producing the same response as the substance under study. The effects of such factors may be discerned through careful design and analysis.
Keywords:  one
One who confounds.