Backend
also: back end, back-end, backends
The behind-the-scenes part of an application that users don't see. It stores information, applies the rules, and does the work the screens ask for.
The part of an application that runs on servers rather than on the user's device. It handles business logic, reads and writes the database, checks permissions, and exposes APIs that the frontend calls.
In a restaurant, the backend is the kitchen. Customers never see it, but it's where orders are checked, food is cooked, and ingredients are stored.
Important rules, like who can see what or how much something costs, belong in the backend. If your agent only puts them in the frontend, people can get around them.
I'll add the check on the backend too, so users can't skip the payment step by editing the page in their browser.
Make the discount limit a backend rule, not just something the page checks. Users shouldn't be able to bypass it.
Thinking the backend is the database. The database stores the data. The backend is the code that decides what to store, what to fetch, and what to allow.