The highest and lowest price of a security during the first few minutes of daily trading activity.
(see also
Initial Balance)
This is mostly used for an instrument that has an underlying cash market with a defined opening period eg stock markets. So in the case of the E-mini S&P 500
Futures Contract (Product Code ES) a trader may use the first 5 minute period of the cash (S&P 500 index) market for the opening
range, being 9.30 am to 9.35 am USA ET (
EST UTC-5:00,
EDT UTC-4:00). The highest and lowest prices in the opening range time period are referred to as the opening range high or opening range top and opening range low or opening range bottom.
Read more: Opening Range Definition | Investopedia
https://www.investopedia.com/terms/o/opening-range.asp
