Have Christians redefined faith?

If you say that Christians have redefined the word, then, of course you are assuming that it initially had a different meaning and Christians changed it. 

I think the term does indeed have several meanings. However, if you want to communicate with Christians to try to show them what is wrong with what they believe, then you really can't argue that the fact that they use the word "faith" means that they are being irrational and they know it. 

An example of someone like this would be this quote. 

Faith is clearly NOT a belief in an unknown or unrealized proposition that is SUPPORTED by the evidence, because if that belief was supported by the evidence, it ipso facto does NOT REQUIRE Faith.

http://meaningwithoutgodproject.blogspot.com/2010/10/faith-last-refuge-of-irrational.html

You can argue that theistic beliefs are in fact irrational, but you cannot argue that because Christians use a word that is sometimes used to denote irrationality, they view themselves as irrational. They are not going to agree to this definition. 

Here's C. S. Lewis: I am not asking anyone to accept Christianity if his best reasoning tells him that the weight of evidence is against it. That is not the point at which faith comes in. But supposing a man's reason once decides that the weight of the evidence is for it. I can tell that man what is going to happen to him in the next few weeks. There will come a moment when there is bad news, or he is in trouble, or is living among a lot of other people who do not believe it, and all at once his emotions will rise up and carry out a sort of blitz on his belief. Or else there will come a moment when he wants a woman, or wants to tell a lie, or feels very pleased with himself, or sees a chance of making a little money in some way that is not perfectly fair; some moment, in fact, at which it would be very convenient if Christianity were not true. And once again his wishes and desires will carry out a blitz. I am not talking of moments at which any real new reasons against Christianity turn up. Those have to be faced and that is a different matter. I am talking about moments where a mere mood rises up against it.