We started working with built-in functions.

“In information technology, the term function (pronounced FUHNK-shun) has a number of meanings. It’s taken from the Latin ‘functio’ — to perform.
1) In its most general use, a function is what a given entity does in being what it is.
2) In C language and other programming, a function is a named procedure that performs a distinct service. The language statement that requests the function is called a function call, Programming languages usually come with a compiler and a set of ‘canned’ functions that a programmer can specify by writing language statements. These provided functions are sometimes referred to as library routines. Some functions are self-sufficient and can return results to the requesting program without help. Other functions need to make requests of the operating system in order to perform their work.
3) In mathematics, a function is a relationship between two variables called the independent variable and the dependent variable. The dependent variable has at most one value for any specific value of the independent variable. A function is usually symbolized by a lowercase, italicized letter of the alphabet, followed by the independent variable in parentheses. For example, the expression y = f ( x ), read ‘y equals f of x,’ means that a dependent variable y is a function of the independent variable x, Functions are often graphed, and they usually appear as lines or curves on a coordinate plane.
4) In a hardware device, a function is one complete physical movement that has a discernible consequence relative to the device’s purposes. In a printer, for example, this might be a carriage return or a line feed.”
http://whatis.techtarget.com/definition/function

Download the class notes for day 3.