I know this isn't the technical definition from any official standpoint, but I personally define gifted as a minimum of 99th percentile on a standardized test. Beyond that, there is a certain internal spark or drive that is also needed.

It is really a matter of perspective. In a university town where most the kids are very bright and have parents with phDs, it isn't unusual to have an entire 1st grade class scoring at LEAST 90th percentile on a (nationally) standardized test. So from that point of view, 99th percentile as the bare minimum to be considered gifted makes sense. Compared to a broader population, most of these kids might seem gifted, but within this little academic nerd world bubble they just seem average.