Aside from the physical definition, as many have mentioned above, a "cellar" indicates an inclination to horde wine. 6 bottle or 6,000. I'm not sure it matters.
Two possible definitions of a cellar, based on Steven's neat insight and others prior to it:
"Any storage facility at home, containing more bottles/cases than the owner might reasonably expect to drink next weekend or remembers drinking last weekend."
"A substantial and morally unjustifiable number of bottles/cases/pallets stored at home, with friends or in a secretive network of Cold War bunkers spanning the country. The owner purports to have have a 'fairly good idea' of their holding — however any figures suggested are invariably incorrect (not least because their CellarTracker account was last updated in 2011), while their partner/accountant/solicitor is often led to believe that the true count is substantially lower. A spurious argument is sometimes made that high value bottles will be sold 'one of these days', but so far to little effect..."