My young friend the Lazy Disciple is right in noticing the fact we commonly use certain words in a meaningless way.
He pointed out saeculum.
I need to point out a couple of heavier ones: religion and faith.
People might think that I am presumptuous. But I am actually amazed by how even the great majority of good Catholics use these words as if there were no problem at all.
Two are the cases:
1) They use them by meaning the right thing, without realizing that those who listen understand them in a totally different way. And I mean listeners who very often are not precisely friendly toward what they want to say by those words.
2) They end by using them in the meaning become current, which is that of their opponents.
Granting or not granting the meaning of words is, as I previously said, a question of rhetoric. We see it when we read Plato's dialogues, where Socrates, to face the self-styled sophists, teachers of the art of persuation, i.e. rhetoric, plays their same game. You claim you know what is knowledge, virtue, justice..., Socrates says; very well, tel me what you mean by them, give me your thesis, a definition to take as basis of discussion, and I'll see if I can grant it. In any given defintion, then, he picks up another word, about whose meaning he enquires, and so on and so forth; until he comes to the point in which they say something in contradiction with the original thesis, which was finally to be abandoned.
So we should beware not to fall into our opponents' traps by granting what we don's need to grant.
I often hear for example faith defined ad "belief in something". Are we sure that it is that what it means? I am rather sure of the contrary. As I am sure that by granting such a definition we put ourselves in the impossibility of giving reasons for what we believe.
The latin fides, from which faith comes, means something eminently interpersonal: the "credit" someone enjoys near somebody else.
Only in a derivative sense faith becomes the credit that we grant to what he says, and finally belief in this.
Even in this case, belief doesn't means anything extraordinary: just the persuasion of the truth of something. I can say for example that I believe in the theory of relativity or in quantum mechanics.
It is in no other way that I say that I believe in the Church's teachings.
As far as religion goes, I'll have to come back to it. The word has become in fact one of the main stumbling blocks to "true religion".