This is the 2664th most frequent Japanese word.
正当
"Justifiable" or "legitimate."
Adjective usage with the meaning of 'legitimate' to describe the reason as justified and proper.
彼の行動は正当な理由によるものだった。
His actions were based on legitimate reasons.
Noun usage indicating the state or quality of being legitimate or justified.
正当性を証明する必要があります。
It is necessary to prove legitimacy.
Adjective usage emphasizing the validity or righteousness of the opinion expressed.