The Truth Project takes as its definition of truth a quote from Noah Webster’s 1824 dictionary:
that which is in conformity to fact or reality
(Missed a heck of a trick by not sticking with John 14:6, but anyway…)
Returning to the Webster definition for the moment, the advantage is that it ties “truth,” “fact” and “reality” together. This keeps someone from playing silly word-games where they talk about how truth is different from fact, and so on. But when you insist on the Webster definition as a way to say that there’s such a thing as truth, and that it’s not relative, a savvy unbeliever will ask you to define “reality,” and before you know it, you’re mired in a debate about post-foundationalism, postmodern epistemology, social construction of reality, reliability of sense perception, optical illusions, misperceptions, and the like. Messy.
Navigable, if you keep your head, remain humble, ask for clarification, and keep your eyes firmly fixed on Scripture, but very, very messy.
Tactically, there’s a better option. The Webster definition has the same essential meaning as a famous definition of “truth” given in one-syllable words (Socrates by way of Plato by way of C. S. Lewis, if I remember correctly):
He who says of what is, that it is, or of what is not, that it is not, tells the truth;
he who says of what is not, that it is, or of what is, that it is not, lies.
Now, because it has the same sense as the Webster definition, it’s vulnerable to the same sort of attack. It’s just that since it doesn’t have the words “fact” or “reality” in it, he can’t ask you to define “fact” or “reality.” Instead, the attack comes at the word “is.”
That’s right. Your savvy interlocutor is going to have to ask you to define “is.”
And thanks to a certain former president, you get to say, “So it all depends on what the definition of ‘is’ is? Really?”
That presidential statement has become, in our culture, a universal symbol for a liar playing stupid word games in order to avoid facing the obvious truth. Milk it for everything it’s worth: “Answer a fool according to his folly, lest he be wise in his own conceit.”
Full Contact Christianity 
Brilliant!