1. [v]  lower the pitch of (musical notes)
2. [v]  become flat or flatter; "The landscape flattened"
3. [v]  make flat or flatter; "flatten a road", "flatten your stomach with these exercises"

1. \Flat"ten\, v. t. [imp. & p. p. {Flattened}; p. pr. &
   vb. n. {Flattening}.] [From {Flat}, a.]
   1. To reduce to an even surface or one approaching evenness;
      to make flat; to level; to make plane.
   
   2. To throw down; to bring to the ground; to prostrate;
      hence, to depress; to deject; to dispirit.
   
   3. To make vapid or insipid; to render stale.
   
   4. (Mus.) To lower the pitch of; to cause to sound less
      sharp; to let fall from the pitch.
   
   {To flatten a sail} (Naut.), to set it more nearly
      fore-and-aft of the vessel.
   
   {Flattening oven}, in glass making, a heated chamber in which
      split glass cylinders are flattened for window glass.
   
   

2. \Flat"ten\, v. i.
   To become or grow flat, even, depressed dull, vapid,
   spiritless, or depressed below pitch.
   
   

To remove structural information, especially to filter something with an implicit tree structure into a simple sequence of leaves; also tends to imply mapping to flat ascii. "This code flattens an expression with parentheses into an equivalent canonical form."

[jargon file]