Paper Wallets
A paper wallet is a private key (or seed phrase) printed or written on paper and stored offline.
In depth
It is fully immune to online attack but vulnerable to fire, water, fading ink and simple misplacement. Spending from one usually means importing the key into software, which exposes it. Today paper is mostly used as a backup for a seed phrase rather than as a primary wallet.
Why it matters
It shows that "offline" is only half the story — physical durability and access matter just as much.