What is the interpretation of a "collider" within graphical causal inference, in the potential outcomes framework? I have recently been reading more on graphical causal models and the term collider often comes up. I am wondering if there exists an interpretation for what it is in the potential outcomes framework. 
 A: I find the definition in Hernan & Robins's forthcoming book (Causal Inference pg 101) to be useful

[Colliders are] a common effect of two other variables in the diagram ... We will use the term selection bias to refer to all biases that arise from conditioning on a common effect of two variables, one of which is either the treatment or a cause of treatment, and the other is either the outcome or a cause of the outcome.

There several example causal diagrams show how it is easier to approach "colliders" from a graphical persepective. You can also frame colliders in terms of single-world intervention graphs (SWIG) which explicitly links the causal diagram and potential outcome approach. 
Let $A$ be the treatment of interest and $Y(a)$ be the potential outcome $Y$ under treatment $A=a$. Therefore, a collider would be $C$ would be written in a SWIG as $C(a, Y(a))$, which indicates it caused by $A$ and $Y(a)$ (i.e. a descendant of both $A$ and $Y(a)$). Conditioning on the collider $C$ in the analysis introduces selection bias, as defined by Hernan and Robins
