The depends entirely on what you rely on to perceive reality, your mechanism, per se.
You, of course, can use multiple mechanisms.
The most reliable mechanism, that has been presented thus far, is science.
So it seems logical to base 'what is reality' on what science presents to us.
Not knowing is a huge part of philosophy behind reality, as well.
Science can not explain everything, as should be obvious.
So for the things science cannot explain there are those who subscribe to different philosophical ideas.
Spiritualism, ultimate reality, solipsism, and so forth. All are equally unevidenced, and have no place with
knowing anything.
(Btw, my definition for reality is provided via the default position.
I believe that reality is simply what it is, and our perception of it should be based on what we know.
Not entirely on ideas without evidence.)
I should probably put this here just for reference.
Reality