A value that is already "built in" to a function.
Example: if a function that works out the height of a tree is h(year) = 20 × year, then "year" is a variable and "20" is a parameter.
Parameters can be changed so that the function can be used for other things.
Example: A different tree might have a growth rate of 30 cm per year, and its function would be h(year) = 30 × year
We could even make it more general by writing
h(age; rate) = rate × age
and in this case a semicolon (;) is used to separate the variable(s) from the parameters(s)