This one is short and sweet my favorite bibical definition of faith

Hebrews 11:1 KJV

Now faith is the substance of things hoped for, the evidence of things not seen.

NIV

Now faith is being sure of what we hope for and certain of what we do not see.

Message

The fundamental fact of existence is that this trust in God, this faith, is the firm foundation under everything that makes life worth living. It's our handle on what we can't see. The act of faith is what distinguished our ancestors, set them above the crowd.