IDIOMS

saddle

(engineering) A part, such as a flange, which is hollowed out to fit upon a convex surface and serve as a means of attachment or support.

Example: A seat (tack) for a rider placed on the back of a horse or other animal
An item of harness (harness saddle) placed on the back of a horse or other animal
A seat on a bicycle, motorcycle etc
A cut of meat that includes both loins and part of the backbone
A low point, in the shape of a saddle, between two hills.
1977, John Le Carré, The Honourable Schoolboy, Folio Society 2010, p. 483:
  With Lizzie leading, they scrambled quickly over several false peaks towards the saddle.
The raised floorboard in a doorway.
The clitellus of an earthworm.