The horizontal bar in a fraction is called a vinculum or, informally, a "fraction bar".
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fraction bar
The fraction form is just that - 4/7. You would usually use a horizontal bar to separate the 4 and the 7.
Now why do you suppose we say "over" in that description ? Could it be because the fraction has the '5' written 'over' the '12', with a horizontal bar between them ?
vinculum
12 parts are in a fraction bar