I am currently writing a paper, which starts with an "Introduction" which is followed by a "Definition" section. I plan to briefly explain a technical term in "Introduction" and define it more specifically in "Definition". According to APA, I need to italicize technical term if they appear for the first time. So am I correct that in this case, I only need to italicize the technical term in "Introduction" once?
In general, you should italicize a technical term when (or right before) you define it. And you should define it when it appears the first time. If you don't need to define or explain it, then you are implicitly assuming that your audience is familiar with it, in which case you don't need to italicize it. The difficulty arises here because you are essentially defining it twice, once informally and the second time formally.
I would recommend italicizing the first use in each of the sections. Clearly, you should italicize it the first time you use it. But if you are defining a lot of other terms in the "Definition" section, leaving it unitalicized might make it look odd next to all the other italicized terms you are defining.
This is not what the APA rules actually say, but I would assume that they also believe that informed decisions in exceptional cases should override their rules where it is appropriate.
Another thing you could do is say something like
we now give a more technical definition of thingamabobs
in the "Definitions" section, and implicitly point out that you've already defined it, which is why you're not italicizing it.