Two-dimensional entity
A two-dimensional entity was a lifeform that was known to exist in a single plane only. As such, they had length and width, but no height. These lifeforms could exist in space and generally stayed within the same plane as others. 

Upon the discovery of the two-dimensional beings near the T'lli Beta system, Captain Jean-Luc Picard of the  for the discovery of such beings "fascinating", leaving his with "so many questions", such as "How can a two-dimensional entity have access to a three-dimensional universe? And are they aware of us?" ()
Lifeform classifications