Lava viscosity is a measure of a lava's resistance to flow, which plays a critical role in determining the behavior of volcanic eruptions as well as the shape and type of volcanoes formed. It is primarily influenced by temperature, chemical composition, and gas content, with higher viscosity lavas leading to more explosive eruptions and the formation of steep-sided stratovolcanoes, while lower viscosity lavas result in gentler eruptions and broader shield volcanoes.