How should I deal with need-to-define terms in introduction?
There are at several possible ways, actually. 

First is to avoid the yet-to-be-defined term in the intro and use a descriptive phrase instead. This assumes the term isn't frequently used in the intro. 

A second way is to footnote the definition or an pointer to it when it first appears in the introduction. Putting the definition itself in the footnote repeats it, but a pointer doesn't require that. 

You could also just add "... frytopper (defined in the next section)..." at the first appearance. 

Yet another would be to write the intro as: "... frytopper (italicized terms defined in the next section)...". For this you would only need the pointing phrase at the first of the later-defined terms, but just italicize all of them - at least for the first appearance. 

Some of these require the reader to hop about a bit, but it should work.