At a recent talk I attended the Secular Humanist speaker dismissed faith as simply believing something without reason - or even against reason. Religious friends, on the other hand, often talk about their belief in the existence of God as a matter of faith. It seems to me that both of these positions are mistaken in various ways.
It was Wittgenstein who coined the idea that if you want to know the meaning of a word then you should examine how it's used in everyday life: there is no source of meaning other than collective usage.
In the case of faith I think it is beholden upon us to look at how it is used in everyday cases in order to truly understand what it means. Perhaps the best explication of faith is one I recall from my Sunday School days. The story is told of the great Victorian tight-rope walker Blondin who crossed the Niagara Falls on a tightrope in the late 1850s. On one occasion he walked across blindfold, pushing a wheelbarrow. Having performed this feat he turned to a bystander and asked him if he believed that he, Blondin, could perform the feat again, but this time with a load in the barrow. The onlooker had no hesitation in saying "Yes - of course you can do it!". At this Blondin said, "Then jump in the barrow and I'll take you across". Predictably the onlooker demurred.
I think this story brings out several important features of faith. Firstly, that faith is a practical matter. It is about commitment to action - even if that action is simply waiting passively and patiently for something to happen. Belief, in contrast, is an assertion that so-and-so is the case. It does not necessarily entail action. It is therefore quite consistent to say that one believes in God but has no faith in him. I expect that many people who assent to Theistic belief in opinion surveys are simply of the opinion that God exists but have no personal faith or trust in him as such. Indeed, this must be the attitude of Satan in the Book of Job as he chats to God in the heavenly realm quite a bit. He obviously is convinced that God exists but it would be absurd to attribute faith to him on that basis.
Sometimes the distinction is made between 'belief-that' and 'belief-in'. To 'believe-that' is to assent to the truth of a statement while 'belief-in' is to take a risk on it and commit yourself to acting on that belief. It is one thing to say that you 'believe-that' God exists (merely assent) and quite a different thing to say that you 'believe-in' God as your saviour (making a life commitment or having faith). However, to 'believe-in' logically presupposes 'belief-that'. Faith is a transitive verb and always implies a person or state of affairs in which one has faith. My religious friends are therefore mistaken, I would argue, when they say they have "faith that God exists". God's existence is a matter of belief and not faith. Faith can only be exercised after belief has first been established.
But my Humanist friend is also wrong in her analysis of faith, for if there is logically a state of affairs that you have faith in then one can always give reasons for one's faith. In a weak sense then, faith can be said to be rational - although it does not follow from this that one has good or sufficient reasons for one's faith. That is something that has to examined on a case-by-case basis as there are no universal rules by which you can determine whether any action is rational.
Madame Butterfly, in the eponymous opera, has faith that Pinkerton will return to be her husband and to be a father to their child. She has good reasons to trust him but is sorely betrayed at the end of the opera when he arrives with his new wife to claim the child and take it away. Butterfly's faith does not pay off in the end but that does not render it irrational. She had good reasons from her point of view, to trust Pinkerton. However, it demonstrates that it is always possible to describe a state of affairs that amounts to one's faith either succeeding or failing to pay off. Indeed, I would want to say that unless you can state up-front what faithfulness or betrayal would amount to in a situation, then your claim faith has no content.
Faith is always a matter of judgement in the face of incomplete information. Faith is therefore an ineradicable part of human life because we frequently have to commit ourselves to action without certainty. But it is only a necessity, never a virtue.