Defect vs bug
In everyday use, defect and bug mean the same thing: software behaving differently from how it should. Formal testing vocabulary distinguishes them, using error for the human mistake, defect for the flaw it leaves in the code, and failure for what a user actually sees.
The longer version
The informal answer is that there is no difference. Most teams use bug and defect interchangeably, and no one is confused. If someone asks you to file a defect, they mean file a bug.
The formal answer comes from testing terminology, where the words separate a cause from its consequences. An error is the human mistake, a developer misreading a requirement. A defect, or fault, is the flaw that mistake left in the code. A failure is what happens when that flaw is executed and something observably goes wrong. One error can leave several defects, and a defect that no one ever executes never becomes a failure.
The distinction earns its keep when you are describing where a problem came from rather than what it looks like. A postmortem that says the failure was a timeout, the defect was an unbounded retry loop, and the error was a misread requirement about idempotency has said three useful things. Calling all three a bug would say one.
In practice, use whichever word your team uses, and reach for the formal vocabulary when you need to be precise about cause versus symptom. Regulated industries and formal QA processes tend to require defect because their documented process defines it.
Common questions
Is a defect the same as a bug?
In everyday usage, yes. Formal testing vocabulary separates them: an error is the human mistake, a defect is the resulting flaw in the code, and a failure is the observable wrong behaviour a user encounters.
Which word should a bug report use?
Whichever your team already uses. Consistency matters more than the distinction, because a queue that mixes both words for the same thing is harder to search. Teams working under a documented QA process usually have the word chosen for them.
Why do some teams say defect instead of bug?
Because their documented process defines the word. Standards bodies, regulated industries and formal QA departments write procedures in the error, defect, failure vocabulary, and a defect is the tracked artefact those procedures describe, with a required set of fields and states. Teams that adopt such a process inherit the word. Outside one, bug is the everyday term and nobody is confused by it.
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