In a spurious relationship, the association between two factors occurs because each is independently associated with some third factor.
The adjective spurious means seemingly associated with/similar to/corresponding to something without having the same qualities. It is often used in a broader sense as a way to say "wrongly or deceitfully attributed to" something.
A spurious relationship or correlation is made when 2 variables are associated due to a third, unprovable factor (sometimes referred to as the "lurking variable").