A "non-invertible" function is one that cannot be reversed to find its original input from its output. In mathematical terms, if a function maps an input x to an output y , a non-invertible function does not allow you to uniquely determine x from y . This often occurs when multiple inputs produce the same output, making it impossible to trace back to a single original value. An example of a non-invertible function is the squaring function, where both 2 and -2 yield the same output, 4 . In contrast, an "invertible" function, like the linear function f(x) = 2x , allows for a unique output for each input, enabling the retrieval of the original input from the output.