To me, colluvium—at least conceptually—is pretty simple. When soil or sediment is eroded (or mobilized via mass wasting) from a hilltop or hillslope, moved downhill, and redeposited before reaching a stream valley, then those deposited materials are colluvium.
But not everyone shares my perspective. Many soil scientists and engineers, for example, restrict colluvium to deposits associated with mass movements. Some geomorphologists attach additional criteria beyond those of my simple definition. This issue is important beyond basic issues of scientific communication, because the identification and measurement of colluvial deposits is critical for studies of sediment budgets and mass balances of hillslopes and drainage basins, and for understanding regolith development and pedogenesis.
Bradley Miller is taking on this terminological, geomorphological, and pedological conundrum (see his blog post on the subject here). Along with Jerome Juilleret, he has developed an interactive poster that outlines the varied definitions and links you to an online discussion of colluvium definitions. And Miller and Juilleret have developed an online questionnaire on distinguishing between colluvium and alluvium. I took it, and I think you should, too. It is actually fun, at least if you are a geomorphology or soil nerd, and seems well designed to generate some useful perspectives on how geoscientists and environmental scientists perceive and define colluvium.
No matter what definition you use, identifying & measuring colluvium in the field can be labor-intensive!