The key step in Konsetvich's proof of quantization of possion manifold is the so-called formality theorem where 'a formal complex' means that it admits a certain condition. I wonder why it is called 'formal'. I only found the definition of Sullivan in Wiki 'formal manifold is one whose real homotopy type is a formal consequence of its real cohomology ring' but still be comfused because most of articles I found contain the same sentence only and I cannot understant the meaning of 'formal consequence'. Does anyone know the history of this concept?