Assume that the following text are excerpts from a book ...
Book: Fun with Math
Chapter 1: We will learn about lots of numbers like integers, real, even numbers and odd numbers.
Chapter 2: There are two kinds of numbers: even numbers and odd numbers.
Chapter 3: An even number is a number which is divisible into two equal integers.
Of the three locations that even number appears in, where is it recommended to italicize it? Is it when it is mentioned (chapter 1), described (chapter 2) or when defined (chapter 3)? Or is it preferred to italicize it in multiple or all appearances?
A more general question is: is a term italicized when it first appears or when it is actually defined? The locations of these two might be far apart in the text.