A function is a relationship between sets of numbers. The first set of numbers determines the value of the second set of numbers.
The value of the first set of numbers corresponds to one and only one value for the second set of numbers.
A linear function is a function that represents a straight line.
A function can be written in different ways:
As an input/output table
In words (such as "the output is two more than the input")
As an equation (such as, y = x + 2)
As a graph