Spatial spillovers are the effect that research and development of individual business' in a geographical cluster, like Silicon Valley, or Taiwan’s Hsinchu Science-based Industrial Park has on the whole. The investment made on R&D in each of the firms is to acquire private knowledge to increase their productivity and profit - in the process, the technology spills over to other firms in the geographical region and becomes social knowledge, which has the effect of improving the productivity of all firms. It is hard to measure. You may find the pdf file in the URL helpful.