Vertical line test
From AoPSWiki
The vertical line test is a way of determining whether or not a plotted graph is a function.
The vertical line test states that a relation is a function iff no vertical line intersects the graph in more than one point.
This is because a function cannot have more than one output for any one input.
For example, 
is a function because any vertical line intersects it in, at most, one point, while 
is not a function (try the line 
).
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