noun
written challenge; letter of defiance
A formal written document declaring a challenge, often used in martial arts, sports, or dramatic confrontations.
ライバルに挑戦状を送りつけた。
He sent a written challenge to his rival.
挑戦 is the act of challenging or the challenge itself, while 挑戦状 specifically refers to the written document or letter containing the challenge.
Compound of 挑戦 (challenge) + 状 (document, letter). The exact historical derivation is uncertain, but the word follows a common pattern for formal written notices.