1. Inadequate in amount or degree
   "lacking in stamina";
   - deficient, wanting
    
2. Nonexistent
   "her appetite was lacking";
   - absent, missing, wanting
    
3. Not present or included (despite being expected, required or desired)
   "lacking in service and professionalism";
   - bereft, missing, devoid, wanting

1. Be without
   "This soup lacks salt";
   - miss
    
2. To be insufficiently prepared
   "He lacked on his presentation and it showed"

See also: inadequate, inexistent [rare], nonexistent, unequal

Antonym: feature

Encyclopedia: Lack, David