taru-adjective, adverb taking the 'to' particle
Archaic literary adjective/adverb describing something that is plainly visible or unmistakably clear. Used in classical-style writing; in modern Japanese, 明らか or 明白 is preferred.
彼の過ちは昭然たる事実である。
His mistake is a manifest fact.
証拠は昭然としている。
The evidence is clear.