I'd been waiting til DD asked/noticed and she did just the other week, when she wondered why she was first to finish a course she was attending for 9-12 year olds (she's 7). It was actually very interesting to see the 'aha' moment when I explained

I did use the 'smart' word, but mostly 'thinks differently', 'has interesting ideas' and 'learns fast'. And said the term for all that is 'gifted', and that's what her pullout day is for. She'd been thinking it was just a fun break from school, which is also true

We haven't discussed actual IQ yet