Vertical Line Test
Note that in the top graph, a single vertical line drawn where the red dots are plotted would intersect the curve 3 times. Thus, it fails the vertical line test and does not represent a function. Any vertical line in the bottom graph passes through only once and hence passes the vertical line test, and thus represents a function.
Source
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"Vertical line test."
https://en.wikipedia.org/wiki/Vertical_line_test#/media/File:Vertical_line_test.png
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