Code minification is the removal of unnecessary characters from source code. Mini, when used as a prefix, means small. The goal of minification is to make source code "smaller" and improve performance. Conceptually, minification is similar to compression. Unlike code that has been compressed, however, minified code can still be worked with -- without requiring the extra step of decompression.
Minified code may not be as rich in features or have the documentation that traditional code does, but the reduction of features has the benefit of reducing the attack surface. Therefore, intercepted or compromised minified code may not as easy for a hacker looking to find an engineering flaw to exploit.
Popular minification tools for Web development include YUI Compressor, Google Closure Compiler, UglifyJS and JSMin. In some areas of programming, minification is also known as de-obfuscation.