A confounding factor (or confounding variable) is an unseen variable that is also correlated with two other variables. The confounding factor is one of the most important reasons correlation does not equal causation, as the causality could be the confounding factor, and may well be counter-intuitive. These have to be examined and controlled in experiments and statistical studies.
Let us suppose for the sake of the argument that breast cancer rates in a country are correlated with abortion rates but that does not necessarily mean that there may be a causal link between the two. There is confounding factor in that both of these are correlated with the affluence of the country. In affluent countries people tend to live longer and by living longer they are more likely to develop breast cancer (sad but true) as it correlates with age far more than any other factor. Additionally in a more developed a country the rate of abortion tends to be higher; so the higher the rates of abortion, the higher rates of breast cancer. Both correlate to the confounding factor, the affluence of the country, rather than each other.