Many still asks the question of faith. Yet the best biblical definition can be found in the Eleventh Chapter of the book of Hebrews.
Now faith is the substance of things hoped for, the evidence of things not seen. KJV
Now faith is assurance of [things] hoped for, a conviction of things not seen. ASV

I am using two versions of the bible here to let us know the other term for SUBSTANCE. It is clear that in American Standard Version is ASSURANCE.

So faith is having the assurance of things that we hoped for.