expression
as soon as; no sooner than; one way or another
Literary or formal expression used after a verb in the dictionary form to mean 'as soon as' or 'whether or not'. Often seen in written Japanese.
彼は家に着くや否や、電話をかけ始めた。
As soon as he arrived home, he started making phone calls.
成功するや否やは、努力次第だ。
Whether you succeed or not depends on your effort.
noun
Rare noun use meaning 'objection' or 'denial'. Mostly found in classical or formal contexts; modern Japanese prefers 異議 or 反対.
彼の提案に否やを唱える者は誰もいなかった。
No one raised an objection to his proposal.