Domain (of a function)

The domain of a function is the set of valid inputs for that function. For example, the domain of concat is the set of all strings, and the domain of the division operation is all non-zero numbers.

To say that a function $f$ has domain $X$ and codomain $Y$, we write $f:X\to Y$ (read $f$ has type $X$ to $Y$). Visualizing a function as a map from an input set to an output set, the domain is the input set (which is the red portion of the image below).