The definition of a number line is a straight line with a "zero" point in the middle, with positive and negative numbers listed on either side of zero and going on indefinitely.
An example of a number line is what a math student can use to find the answer to addition and subtraction questions.
Math. a straight line, theoretically extending to infinity in both positive and negative directions from zero, that shows the relative order of the real numbers
A line that graphically expresses the real numbers as a series of points distributed about a point arbitrarily designated as zero and in which the magnitude of each number is represented by the distance of the corresponding point from zero.