Is a postdoc successful only when completed? Situation: one leaves a postdoc in the middle of the contract to move to another position (postdoc or tenure) at another university. There have been some questions here on SE Academia about leaving a postdoc, but my question is different: may that person still write in her CV that she completed a postdoc? I ask for two reasons:


To me a postdoc is like any other job, but I have heard a postdoc being referred to as 'postdoctoral studies'. In that context, leaving a postdoc before the project finishes might be considered 'dropping out'. 
Long time ago in an office of a professor, next to his PhD degree framed on the wall I have seen a "postdoc certificate", which looks like a diploma.


Hence my questions: is a postdoc successful only when completed (like a graduate program)? Is there a diploma or certificate that postdocs receive upon completion?
"Is a postdoc successful only when completed?"

Short answer: No! 

Long answer: No it's not, mostly because there is no "completing" a postdoc, much like there is no "completion" in academia beyond the PhD unless you are going after a qualification for future promotion e.g. Habilitation in Germany (and possibly other German speaking countries?). 

You are constantly accumulating information and experience, and there is no end to it as far as I can tell. There is always more to read, more avenues to explore, more projects to design, start and complete. Perpetual work... 

There are people who spend less than a year as a postdoc and those that are practically stuck on a forever postdoc situation. When you leave your postdoc is a interplay of several variables:


Availability of funds, or lack thereof
How well suited you are to the research group and/or to the projects at hand (i.e. mutual satisfaction between yourself and your mentor)
What other offers are on the table
Any personal (e.g. family) constraints


I have never heard of a postdoc certificate, and would look very skeptically towards any such. That is not to say that it doesn't exist or that it wouldn't be legit just because I haven't seen or heard it, but it sounds extremely old fashioned and not really relevant anymore. 

At the end of the day, when you write your CV you list your postdoc experience, over a given time period and what projects you have worked with during that period. When a potential employer looks at your CV carefully (in other words if they actually consider you) they might notice if the postdoc experience is too short, or if the projects listed there aren't published. As long as you can justify why you left a postdoc "too early" (whatever that may mean) you shouldn't have any problems

Hope that helps