Inlet

n.
  1. A passage by which an inclosed place may be entered; a place of ingress; entrance;
  2. A bay or recess, as in the shore of a sea, lake, or large river; a narrow strip of water running into the land or between islands.
  3. That which is let in or inlaid; an inserted material.

Inlet is also used adjectively, as in inlet pipe, inlet valve, etc.