Air sac

1. One of the spaces in different parts of the bodies of birds, which are filled with air and connected with the air passages of the lungs; an air cell.

The word meanings were obtained from OPTED(The Online Plain Text English Dictionary), which is based on “The Project Gutenberg Etext of Webster’s Unabridged Dictionary” which is in turn based on the 1913 US Webster’s Unabridged Dictionary (See Project Gutenburg), as a text file.