Diction is simply the issue of making sounds correctly so that the listener interprets them as language. Since singing creates many distortions to the language (pitch, duration, and rhythm) it is impossible simply to sing the sounds the same way you would speak them and expect them to sound like language. The assumption otherwise is exactly why so many amateur singers sound so bad and/or cannot be understood.

KMI Director, Mr. Lee Kikuchi, has developed a wonder vocal method system that teaches the student all the important fundamentals of singing diction, along with how to read music and how to sing correctly.

(Samples of these books can be downloaded from this web site: Kikuchi Vocal Method.)