Backward compatibility
also: backward compatible, backwards compatible, backwards compatibility, backward-compatible
When a new version still works with the older things that depend on it, such as older app versions, older saved data, or other people's code.
A property of a change or release that lets existing users, data, and integrations keep working without modification. It is usually achieved by adding rather than removing, keeping old fields or endpoints alongside new ones, and giving new values sensible defaults.
A new game console that still plays your old game discs. You got the upgrade without losing anything you already had.
You can't make every user update their app at the same moment, and old data stays in your database. Asking for backward compatibility keeps existing users working while you ship changes.
To keep this backward compatible, I'll add the new fullName field but keep returning firstName and lastName until older app versions are retired.
Add the new subscription tiers in a backward-compatible way. Existing subscribers and older app versions must keep working. Tell me which old behavior you're keeping and why.
Assuming the app only has to work with the newest version of everything. Old app installs, open browser tabs, saved data, and integrations keep using the old format for a while.