expression, noun
conclusive evidence; definite proof
Formal expression used in legal, journalistic, or argumentative contexts to emphasize that the evidence is solid and undeniable.
検察は確たる証拠を提示できなかった。
The prosecution could not present conclusive evidence.
確たる証拠がない限り、彼を犯人と決めつけることはできない。
Without definite proof, we cannot label him the culprit.
確実な証拠 is a more common and slightly less formal way to say 'reliable evidence'. 確たる証拠 has a stronger, more literary tone.
確たる is the attributive form of the classical adjective 確し (かくし), meaning 'certain' or 'firm'. Combined with 証拠 (evidence), it forms a set phrase meaning 'conclusive evidence'.