undercross

plural undercrosses

1. engineeringA passage or underpass where a roadway, railway, or infrastructure crosses under another.

   The whole of the undercross is perfect, excepting the bases of the outer columns, and forms a fine superstructure of gigantic support to the light and delicate parts above.

   1800 December, “Description of St Stephen's Chapel”, in THE SCOTS MAGAZINE, volume 62, APPENDIX 1800., page 857

2. knot theorymathematicsThe strand or segment in a knot diagram that passes beneath another at a crossing; i.e., the under‑crossing strand in a knot projection. As distinguished from the overcrossing or over‑strand in knot projections

   If we arrive at an undercross, then we move the branch of the overcross over the rest of the knot up to the end of K.

   2023 January 4, Thomas Fiedler, One-cocycles And Knot Invariants, World Scientific Publishing Company, page 52