In discussions of reasoning or causality, a distal factor is one that is distant from the phenomenon you're studying, as opposed to a proximal cause, which is conceptually or temporally close. For example, consider the question "why the hell is there a node on exploding zombies?" The close, or proximal, reason is "someone wrote some text and pushed the sumbit button." A more distal reason is "he read about exploding zombies somewhere and thought it was an important concept." Still more distal would be "the author was raised to worry about unlikely disasters."

As you can see, distal reasons tend to be more fundamental, but also more prone to interference and complex interaction before being realized. "Distal" may be a relative term, or it may refer to an "ultimate cause" if your system is constructed to allow for that.