There is not one solid definition for the term.
Different sizes, shapes, and designs that look entirely unique from each other can all fall into the category of raised bed. Rather, the term should be considered as a category under which many different approaches to gardening fit. Think of it almost like a movie genre. You have action movies but then under action movies are car flicks and martial arts films. They’re all different but they share the same overall qualities.
We can extrapolate some information from the term itself. The fact that we are speaking about a raised bed means that we are planting our seeds above the ground level. How far above the ground level is up to the individual gardener. Your raised bed may only be raised up a couple feet off the ground but this isn’t always the case. If you have ever seen a bed of flowers attached to a windowsill then you’ve seen another kind of raised bed. Windowsill beds tend to be at waist level (at the lowest) or chest level (at the higher end of the spectrum). So while the height varies quite a bit, the one definite thing that we can agree on is that the bed itself is above ground.
Another feature that is common among the various kinds of raised beds is the inclusion of a frame. Regardless of what the frame is made out of, its purpose is to separate the growing environment inside the bed from the natural environment around it. This frame is packed with nutritious soil and the walls of the frame prevents it from spilling out. Many gardeners choose locations that will allow them access to all four sides of their bed, however this isn’t a feature of the raised bed itself but rather a product that arises from the gardener’s design choices. Raised beds alongside windowsills don’t offer this ability but that doesn’t prevent them from being raised beds, after all.
While a frame is typical, they don’t necessarily need to have a bottom. Many include a bottom to further separate the growing environment from the natural world but this feature isn’t a given. However, a bottom will help to prevent pests from getting into your garden and we’ll be treating the raised beds in this book as if they have a bottom.
So the prime characteristics that we use to identify and discuss raised bed gardens throughout the remainder of the book can be broken down into three features. These are beds that are above ground level, even if only one or two feet. They are designed to create a unique growing space which the gardener has total control of and which is separated from the natural world around it. Finally, these raised beds use a frame along the sides and the bottom in order to keep everything in place. When these three features are present, you have yourself a raised bed garden.
So now that we have this definition in hand, let us turn our attention to the question of why you would want to work with a raised bed in the first place.