What do you call variable that switches on/off the effect of another?

Question

Suppose we have a model

$Y = \beta_1 X_1 + \beta_2 X_1X_2 + \epsilon$

Hence $X_1$ has an effect on $Y$ regardless of the value of $X_2$, but $X_2$'s effect on $Y$ is mediated or enabled by $X_1$.

Please fill in the blank: $X_1$ is a ******** variable for the relationship between $X_2$ and $Y$?

Answer 1

$X_1$ is a moderating variable for the relationship between $X_2$ and $Y$.

Mediation is a related but different concept.