What will testers do if the defect found by tester is not accepted by developer and just ignore the defect by saying that its not a defect?

Check the project specification to see if it is in fact a defect.

Prove to the developer why this is a bug and how it is supposed to work.

If the developer still does not accept that this is a bug, you should speak to someone higher up like the team leader or project manager to ask for their opinion on resolving the issue.

After evaluating against the requirements, if it is a critical bug, then yes it needs to be fixed and if the developer doesn't accept escalate to project manager. If the bug is non critical/ occurs under rare circumstances, then this falls under risk management, categorizing the bug as probably low risk and then managing it and evaluating based on number of occurences when it will need a fix.

1) You have to prove your point by using documented resources, such emails, bussiness doucemnt, system docuemnt, ..

2) if you don't have somthing documented, you have to ask the team leader 

3) if the team leader is not aprove that this issue is BUG, you have to ask the project manger to approve if this issue is bug or not

4) if you dont have a project manger, so any upper mangment level should approve or reject the bug

Check the project Requirements and validate the defect against them , then validate the defect

As test Engineer we provided screen shot of that bug and send it to Developer if again the problem araise we have to check the system enviroinment of both the tester and developer system.....

 

If he (the developer) agrees the issue you reported is there , but he just does not categorize it as a bug then i guess the case should be presented to the "User Experience" or the "Business Analyst" teams to determine whether it is a bug or not (Is the behavior right or not) Now if he can't find the issue (regenerate) it at all and you know how to regenerate it , then you should provide precise steps to regenerate the problem , along with description for your point of view , and any helpful info like screenshots , links ..etc of-course there is some cases where the developer rejects the bug report due to technical limitations , where no solution is possible or there is a solution but it is cost-full to the performance i hope i helped you with my answer , if so please do not hesitate to vote me up best regards

Make a solid proof by using required docs and share it to lead.

Try to provide enough support data or effects generated due to bug.
Adding screenshots, links, small movie etc.
are also beneficial.

As a tester point of view we can give enough evidence regarding that defects .for example we can take screen shot of that defects and attach to bug reporting sheet.
Generally tester report to their Test lead or Manager ..not to developer .
Tester will not argue anything with developer to proof.
Its a responsibility of Test Lead or Manager to accept this defects and report to development team.

1) Attach all the evidences like screenshots and step by step procedure to reproduct the defect.

2) Point out the SRS and BRS details where they need to refer.

3) Still your developer is not convinced, capture / record your findings using a special tools like camtesia (video recording tool)

4) If still he/she is not convinced please escalte your higher level.