When there is a confounder $Z$ for variables $X$ and $Y$, plotting all three variables forms a cloud of points in XYZ-space. The analogue of a regression line is a regression plane with the equation $Y = aX + bZ + c$. The coefficient $a$ is the regression coefficient of $Y$ on $X$ already adjusted for $Z$. It is called a partial regression coefficient and is written as $r_{YX.Z}$. If $Z$ is the only confounder, then $a$ represents the average causal effect of $X$ on $Y$.