Dave Thomas, founder of OTI, says

Clean code can be read, and enhanced by a
developer other than its original author. It has
unit and acceptance tests. It has meaningful
names. It provides one way rather than many
ways for doing one thing. It has minimal dependencies,
which are explicitly defined, and provides
a clear and minimal API. Code should be
literate since depending on the language, not all
necessary information can be expressed clearly
in code alone.