What is exactly an overlong form/encoding?

Reading the Wikipedia article on UTF-8, I've been wondering about the term overlong. This term is used various times but the article doesn't provide a definition or reference for its mean开发者_如何学编程ing.

I would like to know if someone can explain the term and its purpose.

It's an encoding of a code point which takes more code units than it needs to.

For example, U+0020 is represented in UTF-8 by the single byte 0x20. If you decode the two bytes 0xc0 0xa0 in the normal fashion, you'll still end up back at U+0020, but that's an invalid representation.

The Unicode Corrigendum #1 has more information, particularly around table 3.1B.

UTF-8 theoretically allows for different representations of characters that also have a shorter one. For example, you could encode an ASCII character in two bytes by setting the MSBs to zero. The UTF-8 specification explicitly forbids this.