Design is about solving a problem, it's about creating simple and efficient solutions. It can be tangible or intangible, but it always serves its purpose. Design is infinite, can be found in everything natural or artificial and new things can be learnt everyday.
Good design is something that addresses a real problem. As a designer, i feel finding a real problem is a most challenging and important part.
It is not always necessary to come up with new products or system as a solution to a problem. Even a small change in the existing product or system can create a big difference.