As per the definition of a function,which requires that there be a single discrete output (y) for every value of input (x), the graph is not a function because there are two different y-values for a single x-value.
Examples of functions are:
y = x
y = x² - 5
And many other. However, what all of these have in common is the fact that whenever a value of x is input, it gives one value of y only.
