Used; second-hand — referring to items that have been previously owned and are not new.
中古の本屋で珍しい本を見つけた。
I found a rare book at a used bookstore.
その車は中古で買ったが状態は良い。
I bought that car used, but it's in good condition.
中古品は新品より安いことが多い。
Second-hand items are often cheaper than new ones.
In historical contexts, '中古' refers to the Middle Ages; in Japanese history it can specifically denote the Heian period (an archaic usage).
この和歌は中古の作品とされている。
This waka poem is considered a work from the medieval period.
中古の寺院跡が発掘された。
Remains of a medieval temple were excavated.