Colloid may not be a common term in biology these days, but in the early 20th century, colloids were believed to hold the key to the secrets of life. So what is a colloid? According to the Encyclopaedia Britannica it is any substance consisting of particles substantially larger than atoms or ordinary molecules but too small to be visible to the unaided eye; more broadly, any substance, including thin films and fibers, having at least one dimension in this general size range, which encompasses about 10−7 to 10−3 cm. Thus, these are nanoparticles ranging from 1 nm to 10 µm. This upper size limit is relatively high, as other definitions set it at 1 µm or as too small to be studied with a light microscope.
This definition is very inclusive, encompassing not only true globular nanoparticles but also fibers and membranes that may be mere nanometers long in one or two spatial dimensions but much bigger in the others. Many colloids by this definition are not considered to be such by those who study them.