On the 'Properties' section of the Wikipedia page of Null set, it says:
Together, these facts show that the m-null sets of X form a sigma-ideal on X.
I looked for a definition of 'm-null set' on that same page, but I did not find it. I also browsed MathSE, but I only found the post " Null set vs Measure zero set " , which does not answer my question.
What is the meaning of 'm-null set' ?
The article would have been a bit better if it had used $\mu$-null set instead, because it introduces a measure $\mu$ on the space $X$ in the beginning. What is meant is a null set w.r.t. some measure $m$ (or better: $\mu$). It can help if there are several measures under discussion to add the $\mu$- or ($m$-) prefix to "null sets" to avoid confusion. It is good to realise that saying null set means implicitly that there must be an unambiguous measure (from context) we are using to define the notion. E.g. in the context of sets of real numbers it probably means Lebesgue-null set.