It's ill-defined because it lacks context. Asking what's real is too unspecific:
Are you asking in an objectivist, realist, or anti-realist? Fundamentally, you are asking a philosophical question.
For example, some people will hold that electrons exist independently of us.
Other people will hold that electrons do not exist, but that a phenomena that we label as "electron" exists, which we *model* with certain equations.
Still others will deny even that, and hold that the phenomena itself doesn't exist outside human observation.
I hold that there are physical universals that exist independent of human thought, but we as humans cannot know their true nature, we may only model them in our minds using approximations. The approximations *aren't real* They allow us to make predictions of real phenomena, but the model itself doesn't necessarily bear any resemblance to the true nature of the modeled object. (e.g. consider the many different interpretations of Quantum Mechanics. All produce predictions that agree with experimental evidence, but all produce radically different interpretations of what's "real"ly going on. )
As Morpheus said, for us, what's real is what we perceive, what we think and feel. But just because our brains are modeling something, and that there isn't neccessarily one true model, doesn't mean that *thought creates reality*. Clearly, there is something universal being modeled, else no two human beings who evolved separately would come up with the same models, but in fact, we can and do agree on a wide range of universals (existence of gravity, color, temperature, et al)
So, to answer your question, reality exists, but is ultimately unknowable. Reality, as it exists in human thought, is a mental construction, that gets better overtime, a map approximating the physical world, and we get by very well with it.
Ultimately, all we have are concepts that we can communicate to one another, but concepts and true reality are disjoint. The map is not the territory.