A pass leading down from the southern shoulders of the Redhorn into the Dimrill Dale

Crossed the Mountains of Moria above Khazad-dûm

Ran between the Redhorn to the northeast and the Silvertine to the southwest

Dimrill is pronounced 'di'mrill'

Dimrill is archaic English for 'dim streams' ('dim' in the sense of being overshadowed by the surrounding mountains); stair is presumably simply a reference to the fact that this pass led down from the mountains into the valley below

The southeastern part of the Pass of Caradhras, also known as the Redhorn Gate or Redhorn Pass