Finitely additive probability measure

A finitely additive probability measure on a set $X$ is a function $m:\mathcal{P}(X)\to [0,1]$ such that $m(X)=1$ and $m(A\bigsqcup B)=m(A)+m(B)$ for disjoint subsets $A,B\subseteq X$. Unlike a probability measure, it need not be countably additive.