Design: “the careful meshing of human purposes with the larger patterns and flows of the natural world and the study of those patterns and flows to inform human actions” Orr 2002.

The short version.

Design, as a process, has two key defining characteristics:

1. design produces solutions that satisfy a given brief, 
2. design works through iterative cycles of divergent and convergent creative thinking

The longer version.

… and also …

Then there is design thinking and design theory.

There are also many excellent resources to be explored here, including:

1. Herbert Simon, 1970. The Sciences of the Artificial. Design is purposeful. The natural sciences are concerned with analysis to understand what IS. The sciences of the artificial are concerned with synthesis to produce what OUGHT TO BE. They are to do with design.
2. Per Galle’s work on Philosophy of Design