Some definitions of bottleneck specify it as a "severe" limitting component. I usually use the term to refer to any reduction in overall performance due to the limitting effect of a single component. In system designs at work we usually refer to a bottleneck as the slowest component in any system, regardless of its level of restriction (it is still the neck of the bottle afterall), and thus try to balance things from a cost/performance stand point.
As such every system has a bottleneck to some degree, it is only a real big problem in the most extreme cases.
Many people will reserve the term bottleneck to describe the point when the restriction results in effectively zero scaling. Thus no matter what GPU you get there will be absolutely no increase in performance because of the CPU. This is common to all games at ultra low settings, but nearly impossible to attain at any moddest resolution (without going all silly and down clocking the cpu to 10% or something
).
At any rate, it is a pretty subjective term, open for interpretation, and the level of a bottleneck that will be worth worrying about is something that varries greatly from user to user. It is really only important from a cost benefit perspective to 99% of users. This of course assuming a stupid level of system balance is not attempted. (such as using a pcie socket 939 board with a single core athlon 64 and a gtx 295)
If you use wiki as a source half of the definitions stupulate "severe" the other half do not. So to each thier own. Being a metaphore refering to a limiting component and not really technical term it is very much to each their own. Also yes, at any time anyone from this page can change the wiki page to read severe or not as there are almost no references in any of the definitions
.