I never feel comfortable calling myself an "expert" in any skill on the resume, because obviously I don't know everything on any topic. I know nobody knows everything, but I still have this fear that if I am not able to answer a handful of questions on the skill in which I am an "expert", it would create a bad impression. Is there a guideline on when a person can be called an expert in a skill?
There is no universal guideline. This is a signaling problem. Part of the reason there are so many various certifications and degrees is to help solve this problem. With the lack of specific certifications, everyone gets to define their own expert, and the process ends up suffering from the Dunning-Kruger Effect.
While most managers will never admit it, as a society we're really new at trying to figure out who will be good at what in the information age. Unlike measuring who's a good welder (you can easily test a weld), figuring out who will be a good manager or a good programmer is not quite so easy. So we rely on signaling a lot of the time. If you hired 1,000 people out of the following 3 groups, which do you think would produce the most value as a developer?
- High school dropout who has never used a computer and has only worked hourly jobs
- GED-holder with a healthy online profile (github, SE profile, etc.) but no work experience
- BA in Computer Science with 3 years of experience at a tech company and good references
While I'm sure there are people in group 1 and 2 who would be better than some members of group 3, if you're going to hire 1,000 people, you probably want to take from group 3 as much as possible as they have a higher chance to produce value for your company.
That is signalling does.
When people have to evaluate their own ability, they fall victim to the Dunning-Kruger effect. People think they are better than they are. For instance, in the Dunning-Kruger paper, after an exam each student was asked to evaluate how well they felt they'd done. The bottom 25% of test takers actually believed they were in the 60th percentile. People have a tendency to over-estimate their own skill.
So clever employers (and I do hope you are applying to clever employers) are going to give a lot less weight to what you write (expert/intermediate/beginner, etc.), and a lot more weight to the signals that you provide (certifications, degrees, good work history, work samples, accomplishments, etc.).
Who is an expert?
a person who is very knowledgeable about or skilful in a particular area.
You do not have to be the best or know everything. If you are in a great office with brilliant people, you may feel like you are below average compared to them, but the people around you are so far above the average that you are well above average compared to other applicants.
If you have worked at several offices, and you find that you tend to know the most about a certain area/skill, I see no harm in saying that you are an expert in it. If you find that after several interviews (or jobs), you aren't quite as good at a skill as you may have thought you were, then you should adjust your resume to match.
As long as you can explain why you think you are an expert if asked about it, and you have the signaling to back up the statement, the employer likely won't question it.
I think expertise has a close relationship with the experience. If a person has long experience though he is not has educational qualifications with him in relevant field I think still he can be a expert in that area. There is no any measurement to measure "experts" but if he can solve the issues in that area than his colleagues he will become expert there. There may be thing experts also can't answer but they can solve majority of issues.
When turning to your problem, calling your self as "expert" is something you should decide. If you can self confidence to answer all questions (majority of the questions )in your area you can called your self as expert. If you do not have confidence means still you have fear that you may not find out the answer. If you have good enough knowledge and experience explicitly you will get the confidence to called your self as expert. :)