A wingwall is a wall. Go figure. But it's special — it's a wall used to retain soil behind an abutment ("the solid part of a pier or wall, or so on, which receives the lateral pressure of an arch, vault, or bridge pier").
This is important because wingwalls are also components of retaining walls, and interlock with the "facer" or front of such a retaining wall and keep the trail in place too.
The "wing" or "wingwall" is the part of all this that intersects with the rest of the retaining wall at an angle, an angle which can be up to 90 degrees. The wingwall is solidly anchored and assists the facer part of the wall in retaining fill material, helping to prevent "flanking", which is what happens when the retained soil or "fill material" tries to creep around the retaining wall.
Like, the part of the bridge-end that is stuck into the bank, and the short walls that are like wings protecting it. If this definition explains wingwall adequately to you, write me. 'K?