What is soylent?
In the narrowest possible sense, Soylent is the beverage which was invented by Rob Rhinehart and is being marketed by Soylent Corp. But we are specifically interested in DIY soylent, which is soylent that one makes at home. For our purposes the definition we have given is too narrow; we are interested in soylent which is not made by Soylent Corp. It is worth noting that Soylent Corp. holds the trademark on Soylent. For us capital-letter "Soylent" will refer to the product made by Soylent Corp.; and lower-case "soylent" will refer to any other soylent.
Elucidating what exactly soylent is, is not entirely trivial. Typically we envision a beverage, mixed from a non-perishable powder, which provides complete nutrition, and which is reasonably priced, such that a person of ordinary means could afford to live on it as their sole source of food. But thanks to innovations of the community, the definition of soylent as a beverage is no longer catch-all. Soylent can be a brownie, a custard, a porridge, or even a soup.
We can reasonably give the following definition. Soylent is a single food item which provides complete nutrition, and is reasonably priced (in the same sense as before). Anything fulfilling these criteria deserves to be called soylent.
The word soylent is also used by some people to refer to a collection of foods which together provide complete nutrition; for example this and this. To me this seems to be stretching the definition of the word "soylent" beyond meaningfulness. For the purposes of this guide we will assume that "soylent" means a single food item.