The basal lamina is a layer of extracellular matrix secreted by the epithelial cells, on which the epithelium sits. It is often confused with the basement membrane, and sometimes used inconsistently in the literature, see below.
It is typically about 40-50 nanometers thick (with exceptions such as the basal laminae that compose the 100-200 nanometre thick glomerular basement membrane).
It includes proteoglycans.
Read more about Basal Lamina: Layers, Basal Lamina Vs. Basement Membrane