what is the terminology that describe the rate of user access over order in the context of e-commerce
Solution 1:
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.