There's not really a standardized definition for it, necessarily. Generally, it means basically what you would guess. You feel like you're an imposter in your field. It's pretty common among developers, especially self taught, and I've heard of several designers feeling the same way. As a self taught developer myself, this is definitely something I've experienced over the course of my nearly four years of being a developer. So in this blog, I'll be discussing it specifically through the eyes of a developer.
Before going any deeper, I think it's important to mention the polar-opposite: the Dunning-Kruger effect. In short, it's where the 'sufferer' has a very simple, high-level understanding of a (usually) complex topic, and somehow end up thinking that the topic is far more simple than it actually is. As a result, they often consider themselves masters of the topic. They know all they need to know, their knowledge is complete. They have a tendency to over-simplify everything and don't bother digging deeper into whatever topic they consider themselves an expert in. Imposter Syndrome is often quite the opposite. The more you learn about something, the more you realize how much you actually don't know. For the most part, this is a humbling experience. But for some, it leaves them feeling like they're just a fraud, and that the fact that they've made it as far as they have in their respective career is just pure luck.
As a self taught developer, most of my learning came from either reading books or watching/following tutorials online. These guys don't just sit down in front of their computer and unload all of their knowledge in a single sitting, that much is obvious. However, that doesn't mean it's going to be the first thing that crosses your mind. I can easily recall watching these pros sit down and throw together a quick web application for learning purposes, and thinking "Man, this guy knows his stuff" or something along those lines. The reality is, he probably already built that application beginning to end, and he's re-building it real-time while explaining the thought process and the techniques being used. He's already looked up the functions he needed a refresher on, and it's all sitting off-screen.
Whenever one suffers from Imposter Syndrome, it can be easy to feel as if you're alone. But in fact, most if not all design or creative professionals will experience this at some point in their career. To broaden my scope, I recently had a discussion with one of the graphic designers on our team about this subject. I was a little surprised to learn that even he struggles with it from time to time. I found it interesting that a person who's always bursting with creative ideas could feel insecure about his ability. After learning that he has gone through this himself, I asked him about his coping methods. He states "I always remind myself that every project should be a challenge, even the small ones. Every project should be hard and a new opportunity to get better. The fact I feel a pressure to make something great is a good thing. This means that I genuinely still care. And the people that I look up to are successful because they approach every project with this same attitude."
This is going to differ from one person to another, so I can only speak from my own personal experience. Fortunately, Imposter Syndrome isn't as big a deal for me as I've seen in others. Still, it's always in the back of my mind. I've read about it a fair amount, and even listened to a few podcasts on the matter, to find ways to better handle it when it does rear its ugly head. The best advice, at least what's worked best for me, is to avoid comparing yourself to the 'pros' at all costs. By all means, strive to one day be on their level. But if you expect it to happen quickly, you're setting yourself up for failure.