For an, examplary simple, `ElementMesh` 


 p = {{0, 0}, {1, 0}, {1, 1}, {0, 1}, {1/2, 1/2}};
 mesh = ToElementMesh["Coordinates" -> p,"MeshElements" -> 
 {TriangleElement[{{1, 2, 5 }, {2, 3, 5 }, {5,3, 4 }, { 1, 5,4 } }]}];
 Show[mesh["Wireframe"], Axes -> True, AxesLabel -> {x, y}]

[![enter image description here][1]][1]



I would like to select part of the mesh `{x,y}`which fullfills condition `y>=x`

Is there a simple solution? 

Thanks!


 [1]: https://i.sstatic.net/ELxWd.png