**Collinearity** is a linear relationship that is present between two [[Independent Variable|explanatory variables]]. If the [[Correlation|correlation]] between them is 1 or -1, it is considered that these two variables are **perfectly collinear**. Despite it implies the existence of a linear relationship, it does not refer to the [[Causality|causality]] or lack thereof between the two variables. Collinearity is an undesired property in [[Regression|regression models]], since it can lead to skewed or noisy results. Ideally, only one of the two (or more, if there is [[Multicollinearity|multicollinearity]] present) variables should be chosen to be used as explanatory.