It's like defining "charisma" or "star power." You just know-- it's unmistakable once you've seen it firsthand.
This is interesting and probably true. I wonder, then, why educators and parents who have seen "gifted" still call a lot of other things gifted as well. For instance, I know dd13 and a few of her cohort are truly HG+ and they are very, very evidently quite different from the large majority of the GT ided kids with whom they attend school. On the other hand, I still know parents of a lot of other kids who differ dramatically from this cohort as well as educators who still call these dramatically different kids gifted as well.
Then again, I have had educators who are quite bright themselves be pretty upfront with me and tell me that the majority of the GT ided kids are high achievers, not gifted so maybe the ability to recognize the difference requires the individual to be very intelligent him/herself.
