A control variable is a variable that is included in an analysis in order to control or eliminate its influence on the variables of interest. For example, if we are looking at the relationship between having a university degree and smoking prevalence, we might need to consider the impact of age at the same time. Older generation respondents are more likely to smoke than a younger generation. If we control for age, we can see whether graduates are less likely to smoke than non-graduates once age has been accounted for.
Source: SAGE Research Methods.