"Boneless" was the name the Twelfth Doctor gave to two-dimensional beings from another plane. They were able to reduce both lifeforms and other three-dimensional objects to two-dimensions. They were also able to change objects from two-dimensions into three-dimensions after taking a three-dimensional form when the 2Dis reversed its original effects.
History
The Boneless experimented and dissected three-dimensional lifeforms and objects, and eventually learned how to exist as three-dimensional beings themselves, although their transformation into 3-D beings was not perfected, resulting in an appearance where they seemed half 3-D and half 2-D.
The Doctor had hoped that their experimenting on the humans in Bristol was strictly due to some misunderstanding, as the Boneless originated from another dimension, and therefore were unable to communicate in a language that 3-D organisms can comprehend.
The Doctor used his sonic screwdriver and an energy field from the TARDIS to forcibly send them back to their home dimension, without regard to their chances of survival on the return trip, and with a warning to any more Boneless who wish to conquer the 3-D universe, that the Doctor will stop them.(TV: Flatline)