The survey was conducted in February and March and received roughly 478,000 responses, in which people picked their favourite term among 13 options to describe the hottest day.
Japan already has terms for days over 25C, 30C and 35C.
The new term for even hotter weather comes after record-breaking heat hit Japan last year.
Summer 2025 was the hottest on record in Japan – records date back to 1898 – with average temperatures nationwide 2.36C above average.
Temperatures reached 40C-plus on nine days between June and August, with a new national record of 41.8C set in the city of Isesaki.
The cumulative number of extremely hot days also surpassed the previous record set in 2024.
For example, Tokyo recorded 25 days over 35C, compared with an average of just 4.5 days. Kyoto logged 52 days above the same temperature, compared with an average of 18.5 days.
This summer, JMA forecasts a high probability of above-normal temperatures in Japan from June to August.