noun
living witness
A person who has directly experienced a historical event and can testify about it firsthand.
彼は戦争の生き証人として、その体験を語り継いでいる。
As a living witness of the war, he continues to pass on his experiences.
証人 is a general 'witness' (e.g., in court or to a document), while 生き証人 specifically means a living person who can give firsthand testimony about a past event.
Compound of 生き (iki, 'living') + 証人 (shōnin, 'witness').