undercross

noun2

1. •
   A 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
2. •
   The 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