We have all heard people claiming "This is the truth." But what is "Truth"?

The most basic definition of truth is the conformity of the intellect with what the thing perceived actually is. This would be objective truth.
Many in our culture want to make such truth relative, meaning, you have your truth and I have mine.

This is not truth if one's perception does not conform to what it actually is, then one is in error, no matter how convinced they are, and certainly no matter how one feels about it.

People who are color blind are not seeing all the true colors before them. Their perceptions are distorted, even though they might not be aware of it.

When I make a statement about something that does indeed exist, then such a statement could be called "truth".
Any statement that would contradict that statement would then be an untruth.

If such a statement is deliberately made in order to deceive, then that statement is called a lie.

God is the source of all that is, and knows His creation perfectly. He is the Source and fullness of Truth.