What is the difference between a design pattern and an anti-pattern?

A design pattern is a proven solution template for a recurring design question; an anti-pattern is a pattern that is unsuitable, harmful or useless, in plain words "a bad idea".

Both terms are used in software development and in architecture. Knowing the patterns lets an architect reuse solutions that have already worked; knowing the anti-patterns is just as valuable, because many insecure designs are familiar mistakes repeated. Security-by-obscurity, relying on attackers not knowing how something works, is a classic anti-pattern when it is the only protection.