This is a tricky one because if I look at my team, everyone is a specialist. I haven’t recruited them to be a specialist but they naturally become one in certain situations - both in tech skills and soft skills.
There’s also a negative aspect of the term, because in order to be a specialist in something, there has to be something you’re not a specialist in or literally an area you just don’t want to do.
So my stab at it would be a specialist is someone who chooses to focus on a set of skills they are passionate about to become a respected mentor and advisor in those skills.