Hebrews 11:1 tells us what faith "is" and that's good enough for me but apparently not for you.Hebrews 11:1 doesn’t define pistis - it engages Greek reasoning rhetorically. And since the faith-alone system isolates faith so sharply, one might expect it to clearly define biblical faith. But it doesn’t - it at minimum strips pistis of its relational depth and adds words to Scripture to protect itself. Shifting to “faith vs. works” avoids the charge of redefinition by changing the subject.
A definition of faith from the Greek:
http://biblehub.com/greek/4102.htm