FSI stands for Floor Space Index. It is a ratio that measures the total floor area of a building or development to the size of its land plot. In other words, it indicates how much of the land area is covered by the building.
A higher FSI means a building can have more floors or a larger footprint on the land, while a lower FSI limits the amount of construction allowed on a specific plot. FSIs are often regulated by local zoning laws to control building density and ensure adequate open space within urban areas.