UX Glossary · O
Optimistic UI
Showing the expected result immediately and reconciling with the server afterwards, trading honesty for speed.
Optimistic UI updates the interface as though an action succeeded, before the server has confirmed it. A liked post fills in instantly; the request is still in flight. If it fails, the interface rolls back.
It exists because latency is unavoidable and waiting feels bad. Done well it makes an application feel immediate on a slow connection.
What it trades away
Truthfulness about system state. For most of the time, the interface is showing something it does not yet know to be true.
For low-stakes, high-frequency, near-certain actions, that is a good trade. Liking, marking read, reordering a list. Failure is rare and recovery is cheap.
For consequential actions it is the wrong default. Anything involving money, permissions, medical or industrial state, or anything the user will act on elsewhere, should show what is actually true, including a visible pending state.
The failure mode that matters
Silent rollback. If an action appears to succeed and quietly reverts while the user has moved on, they will believe something happened that did not. That is worse than a slow interface, and in a clinical or industrial context it is a safety issue rather than an annoyance.
In practice
The question is what the user will do next based on what they saw. If they might rely on it, show the truth and design the waiting state properly.