This is the 5433rd most frequent Japanese word.
あかし
Proof; testimony; evidence; sign.
あかし (証) is used here to mean 'proof' or 'evidence'.
彼の行動は誠実さの証だ。
His actions are proof of his sincerity.
In this context, あかし indicates a sign or indication of something.
植えた桜の花が咲いたのは春の訪れの証です。
The blooming of the cherry blossoms signifies the arrival of spring.
Here, あかし is used in the sense of a testament, expressing deep emotional resonance.