A more thorough explanation:

Definition: Dead letter refers to two different things:

1. A law or practice that is no longer used or enforced, even though it has not been formally abolished.
2. A piece of mail that cannot be delivered or returned because it has incorrect addresses for both the sender and the intended recipient.

Example 1: The law requiring people to carry a lantern when walking at night is a dead letter, as it is no longer enforced.

Example 2: The post office received a dead letter because the address on the envelope was illegible and there was no return address.

The examples illustrate the two different meanings of dead letter. In example 1, dead letter refers to a law that is no longer enforced. In example 2, it refers to a piece of mail that cannot be delivered or returned due to incorrect addresses.