Question 1039933
This requires that a function pass both the vertical and horizontal line test.
y=x^2 is a function.  But it is not 1:1, for y=4 has two possible x values, -2 and 2.
y=x^2 for x>=0 is 1:1.  It passes the horizontal line test. It is a function, but the domain has been restricted so that x may only be positive.