WordWeb Online

1. Make flat or flatter
   "flatten a road"; "flatten your stomach with these exercises"
    
2. Become flat or flatter
   "The landscape flattened"; "The landscape flattened out"
   - flatten out
    
3. Lower the pitch of (musical notes)
   - drop

Derived forms: flattening, flattens, flattened

Type of: alter, change, change form, change shape, deform, form, modify, shape

Antonym: sharpen