assume I have an e-commerce website.
let num_user denote how many user access my website in a day,
let num_order denote and how many of them make an order in that day.
the question is, what is the terminology describe the rate num_order/num_user?
This is known as the "Conversion Rate".
https://www.optimizely.com/uk/optimization-glossary/conversion-rate/
"The conversion rate is the number of conversions divided by the total number of visitors. For example, if an ecommerce site receives 200 visitors in a month and has 50 sales, the conversion rate would be 50 divided by 200, or 25%."
"Conversion" is a common term in sales - one looks to "convert" an opportunity into a sale.