x1 is a mereological object which underlaps with mereological object x2 via at least object x3 in universe of discourse x4.
There exists an object y in x4 such that x1 is a part of y, and x2 is a part of y; moreover, x3 is such a y (but others may exist). Sometimes, this definition is restricted so that objects such that all things in the universe are part(s) of them are not candidates for y. See also: "pagbrkuclapi", "pagbu".