The exclamation mark in an expression such as x! (read as "x factorial") is a computational recipe indicating that you should take x, along with all positive integers smaller than x, and string them together with multiplication signs. Note that 0! and 1! are both equal to 1. Thus
An expression of the general form xn can be read as "x to the nth power" or "x to the power of n." The quantity n is spoken of as the exponent of x, and the process of raising any number to the nth power is spoken of as exponentiation. As a computational recipe, xn simply means "take n instances of x and string them together with multiplication signs." Note that any number raised to the power of zero is equal to 1, and any number raised to the power of 1 is equal to itself. Thus