Well, you could think of stability in terms of malware (viruses, spyware and adware).
For example, there are more viruses on the Windows platform than there is on Mac OS X. So you could say that Mac OS X is more stable than Windows because there's fewer malware.
A factor as why there's more malware on the Windows platform is because of the market share of operating systems (Windows having greater dominance). Obviously it would be more favourable for virus writers to target the Windows platform.