Buzz
It's what we hear inside our heads when we sing, or better yet: it's what we had better hear inside our heads when we sing.
Yes, I am quite aware of those who intone that the singer should not listen to herself. To them I respond: Do you shut your eyes when you drive? Because that's what—auditorially-speaking—you are asking your student to do when she sings. What this statement betrays is the teacher's lack of knowledge.
The basics?
Human beings with normal hearing process sound in two ways.
Bone Conduction
Air Conduction
We hear bone conduction in our heads and air conduction outside our heads since it travels from our mouths back to our ears.
Which route is faster?
Bone conduction. It's what we hear and feel first. It's also what must be developed in nine out of ten singers.
It is discovered in closed vowels like [e] and [i].
Margaret Harshaw introduced this concept to me during my first lesson with her in 1985. She called it "the buzzy business that never turns off." She also told me that it felt "hard" and would take a long time to get used too. She was right. Her words were underlined when I underwent Tomatis' listening training in Toronto in 1999. There, I listened to Mozart filtered at a high frequency and beamed into my head via bone conduction. The tickle of buzz I heard and felt informed everything I had been taught. To put matters succinctly?
It don't mean a thing if it ain't got that buzz.
That said, we also need the awareness of air conduction. They are inseparable, bone and air. In fact, you can't have one without the other. The old Italian school had a perfect visual metaphor for their relationship.
The bow and arrow.