From CS2800 wiki

The output of a function is unambiguous if there is only one output for any input. It is especially important to check this when the input of the function can be written in different ways. For example, [math]f : \href{/cs2800/wiki/index.php/Enumerated_set}{\{1,2\}} \href{/cs2800/wiki/index.php/%5Cto}{\to} \href{/cs2800/wiki/index.php/%E2%84%95}{ℕ} [/math] given by [math]f(1) := 2 [/math], [math]f(2) := 2 [/math], and [math]f(1+1):=4 [/math] is not unambiguous, because [math]2 = 1 + 1 [/math] so it is not clear if [math]f(2) = 2 [/math] or [math]f(2) = 4 [/math].