What is truth?

truthn
truːθ/
noun
noun: truth
  1. the quality or state of being true.
    “he had to accept the truth of her accusation”
    synonyms: veracity, truthfulness, verity, sincerity, candour, honesty, genuineness; More

    antonyms: dishonesty, falsity
    • that which is true or in accordance with fact or reality.
      noun: the truth
      “tell me the truth”
      synonyms: the fact of the matter, what actually/really happened, the case, so; More

      antonyms: lies, fiction
    • a fact or belief that is accepted as true.
      plural noun: truths
      “the emergence of scientific truths”
      synonyms: fact, verity, certainty, certitude; More

Origin
Old English trīewth, trēowth ‘faithfulness, constancy’ (see true, -th2).