In Jenga, how do we define the "highest completed layer"?
The rules as of 2018 don't refer to the "highest completed layer", but older versions and variants (like Jenga Quake) do refer to it.

The highest completed layer is the highest layer that has had all three blocks placed on it. Recall that when you remove a block, you must place it on the top level until it has three blocks. After three blocks are placed, the next block is placed on the next level.

The current iteration of the rules corroborate this by referring to "the one below an incomplete top level" where older rules would say "the highest completed layer" (emphasis mine).


  Moving in the game Jenga consists of:
  
  1) taking one block on a turn from any level of the tower (except the one below an incomplete top level), and
  
  2) placing it on the topmost level in order to complete it.