WordWeb Online

1. (of the Moon) pertaining to the period during which the visible surface of the moon decreases
   "after full moon comes the waning moon"

1. A gradual decrease in magnitude or extent
   "the waning of his enthusiasm was obvious"; "the waxing and waning of the moon"

1. Grow smaller
   "Interest in the project waned";
   - decline, go down
    
2. Become smaller
   "Interest in his novels waned"
    
3. Decrease in phase
   "the moon is waning"

Type of: decrease, diminish, drop-off, fall, lessen, lessening

Antonym: wax, waxing

Encyclopedia: Waning