The marketing term voice of tone refers to the way or tone of a company's or brand's communication. A definition of tone of voice is meant to line out how communication should reflect attitudes and characteristics and keep them consistent accross different media and publications.
Tone of voice isn’t what we say but how we say it. It’s the language we use, the way we construct sentences, the sound of our words and the personality we communicate. It is to writing what logo, color and typeface are to branding.
(Source: http://uxdesign.smashingmagazine.com/2012/08/21/finding-tone-voice/, 2013/01/03)