Asking this question is about as general as asking the Ab Fab darlings, what is Lacroix (their fav fashion designer). They would respond with a prompt sentence that he is a fabulous fash designer who cannot be easily defined but is influenced by many aspects of life. The answer for what is voice is just as ambiguous.
Most definitions cite that voice means an author can be "heard" through his text. "An author can inject a specific presence or identity into a text, creating an individual style that marks the text as genuine, authentic, and as belonging to a specific author with a specific stance." (Bowden 9) Readers know when they are reading a text that has authentic voice. Think of Shakespeare, Freud, or Walt Whitman; readers know when they are reading these individuals without even seeing the author's name. Bowden found that readers "revoice" (since the author has in some way "voiced" the text in the process of composing) as they read. (Bowden 10) This revoice helps the writer take its place in communication with other voices, including that of the reader.
Voice allows writers the ability to define and locate themselves relative to other discourses, to write themselves by appropriating and rewriting others. (Yancey xix) Yancey also brings up another important issue: voice is also fictional.
Each time we write, it is possible to wrest from the language and from ourselves a new voice that may or may not be truthful or authentic, for what is the source for such an evaluation? Authentic to who we used to be? To who we are now? To which of the current conflicting selves? Perhaps, then, voice isn't so much an authentic or nonfictional, but faithful -- to the current voices composing the writer. (Yancey xix)
What is tantamount to remember from all of this discussion about what is voice is that voice is empowerment. In and out of the writing world, it gives students confidence and competence in writing and problem solving tasks.
definitions from writing professionals and authorities:
and later on, when a paper is full of voice errors