Aletheia (Ancient Greek: ἀλήθεια) is truth or disclosure in philosophy. It was used in Ancient Greek philosophy and revived in the 20th century by Martin Heidegger. It is a Greek word variously translated as "unclosedness", "unconcealedness", "disclosure" or "truth". The literal meaning of the word ἀ–λήθεια is "the state of not being hidden; the state of being evident." It also means factuality or reality. It is the opposite of lethe, which literally means "oblivion", "forgetfulness", or "concealment".