Oh wow this is a big one, with no easy answer!

It is far from a homogeneous, easily defined area. It can refer to both process and product, ranging from the abstract conception of something to the actual plans and processes required to achieve it. Design is also ubiquitous: ‘we all live within a design cluster, that is, immersed in designs of all kinds, which means that design becomes, in Peter Lunenfeld’s words ‘a category beyond categories’ (2003, p.10).

This blog post might help