Bug

A bug is a defect causing software to behave other than intended. The term predates computing and was popularised in the field after 1947, when operators at Harvard found an actual moth in the Mark II relays and taped it into the log, noting the “first actual case of bug being found”.

Why the distinction from a usability problem matters

A bug is code doing something other than specified. A usability problem is code doing exactly what was specified, where the specification was wrong. Both produce the same symptom, which is a user who cannot finish.

The distinction matters because the routes to fixing them are different and the second one has no owner by default. “The form rejects valid phone numbers” gets triaged. “Users do not understand which phone number the form wants” often gets closed as working as intended.

In practice

Users make no distinction at all. They experience a product, not a division of responsibility, and a confusing screen and a broken button both read as “this is bad”.

This is worth remembering when a usability test turns up both. Separating them is useful for routing the work and misleading if it is used to argue that only one category counts.