Hall, who introduced the concept of informal space in the anthropological literature, did so to identify this specific category of spatial experience. In his view, space becomes informal or is created as such 'because it is unstated, not because it lacks form or has no importance... Informal spatial patterns have distinct bounds, and such deep, if unvoiced, significance that they form an essential part of the culture' (Hall, 1966: 105).
space is a flexible entity — a socio-political production — that can be informalized.
— preview of The informal city by Michel S. Laguerre. URL
full introduction: https://link.springer.com/content/pdf/bfm%3A978-1-349-23540-7%2F1.pdf