noun
testamentary document; will
A formal legal document stating one's last will and testament.
彼は遺言証書を公正証書として作成した。
He had his will drawn up as a notarized document.
遺言 is the general term for a will or last words, while 遺言証書 specifically refers to the written document.
Compound of 遺言 (last will) and 証書 (certificate, document).