Teoría general de la urbanización (General Theory of Urbanisation, 1867) by the Catalan engineer Ildefons Cerdà set out a systematic, data-driven theory of how cities should grow, and coined the Spanish term urbanización. Cerdà analysed mobility, hygiene, sunlight and circulation statistically, treating the city as a problem to be solved with general principles rather than ad hoc design. He paired the theory with his celebrated extension plan for Barcelona, the Eixample, an open grid of large blocks with chamfered corners intended to admit light, air and easy movement. The work combined social reform aims with engineering analysis.
It is widely credited with establishing urban planning as a self-conscious modern discipline and providing its founding vocabulary. The accompanying Eixample became one of the most studied and emulated city-extension grids in the world.