Difference between Defect and Bug?

In Six Sigma usage, a defect is what the project defines it to be.  It doesn’t have to be at all related to a bug.
In software test usage, a mistake is something that could compromise the product which was found and corrected by the person who made it.  If someone else in the phase finds it, it is an error.  If it escapes the phase, it is a defect.
–McD
 

Error: Is what occurs when u try to do something which is nonexistent. It is always man made. It has a life span of a phase. If it slips to the next phase its termed defect.
For simplicity, consider that a project is in unit testing phase (generally done by a programmer/ by a white box tester).
The project is “An application of password authentication”. The password needs to have at least 6 characters.
If the programmer mistakes by giving “>” sign instead of “>=”; and if he finds and corrects it, it is as “An error”.
 
Defect: As mentioned before if an error slips from one phase to the next and detected there it is termed as a “Defect”.
Now assume that the above error is carried forward because of some reason in the next phase (Lets say integration), and detected in that phase. It is termed as defect and not

error (As a defect is lack of something). Even this is man made and the life span is more than a phase.
 
 
Bug: A bug is “A deviation from requirement”. The requirements are always defined. So whatever not meeting the requirements is a bug.
With regards to the above example if the defect still persists in the last stages of the project like system testing it will be called as a bug, if detected and the same terminology

continues for the next phases(The reason is that the system test phase is the first phase towards the acceptance testing and its nearer to the customer. It really helps to follow the

same notation for the remaining project and not to confuse the customer with all the above terminology).
Bugs may not be man made, They can be because of the system environment, behavior or maybe because of misunderstanding.
 
One more example for a bug.
Consider a simple cosmetic bug- If the customer wants a word “colour” on the screen but the developer puts “color”.
What’s the problem here. You will say both are same but they are not, when you say “colour” its British English and when you say “color” its American English.
You can conclude that your client is a Britisher. Now tell me, will you like if I twist your language, even if it doesn’t make a lot of difference to us as we follow both dialects; no, it won’t. Everyone is proud of there language, so here, we have an impact on customer relations, its no kidding and as far as I know Britishers are very much proud of there language.
Morale of the story, it is a bug.

Rahul, I am agree with your definition of Error, but would like to present the other way of looking at concept of Defect and Bug…
Defect:- we all know and accept that a Error after completion of its life span turns to “defect”, but how do you visualize that , you never know about the defect until you see the failure, when you encounter the failure, analysis of the failure will lead to the defect/error which has caused it, 
The Bug is this way until you find the defect which leads to Failure it remains a (unknown Defect) Bug, because it’s unknown to us as what is the defect which has caused the failure/Failures.
Prakash

Rupali, read the prior posts in this string; it is explained thoroughly.

bug produce before that goes into production
detect is an error which will come after production .