Two things are commensurable if they can be ordered by some single measure. There is often a real problem about knowing whether this is so: deciding whether two persons' utilities or states of happiness can be measured one against the other, for example. In the philosophy of science two theories are said to be commensurable if the claims of one can be framed in the language of the other. When two theories are incommensurable there may be no neutral standpoint from which to make an objective assessment of the merits of the one versus that of the other.
Philosophy dictionary. Academic. 2011.