I can't think of an adequate answer to that. I could say that it's an idea that would be deemed incoherent with one's other beliefs if it were deemed false (coherence theory of truth), I could say that it's an idea that's deemed useful in applying it to reality, I could say it's an idea that is useful in applying it to reality (the difference between those two is whether truth is subjective or objective), I could say that truth is the state of reality (but then how can a proposition be "true', if reality isn't made of propositions, which are semantical and ideational?), I could say that truth is a state of mind that is properly aligned with external reality (correspondence theory of truth). .

The natural thing for me to do is to take a subjective stance on the nature of truth, but that's not adequate to me because I treat truth in my mind like it's objective and I don't think that's fallatious.

Also, even if I adopted one or more of the definitions above, I think it would be inadequate because there seems to be a transcendental aspect to truth, sort of as if the highest truths are something like religious experiences, i.e. truth goes beyond mere analytical or semantical ideas, and I'm not sure how to capture that aspect in a definition. (Well, maybe the 'alignment' definition works there, but then, it's kind of poor because it doesn't explain the distinction between a 'proper' alignment and an 'improper' one.)