What makes no difference to, has no bearing or influence on, is independent of
something else is said to be irrelevant to it;
the probabilistic sense.
This is an important concept, if only
because in practice and in theory we in fact hold many things and events to be
irrelevant to our present practice or theory, while we may be quite mistaken in our
- often tacit - beliefs about irrelevance.
Note that an important part of most methodologies in the empirical sciences is to control whatever is known to be relevant to the conditions and outcomes of the experiment: One wants to avoid experimental outcomes that do not in fact depend on a hypothetical explanation but on some other ("interfering") factor.
There are more subtle definitions of
irrelevance and independence in