Prince Burridge

I think technically the definition of ska is music where the bass drum lands on the third beat of the bar, and the back beat is not played on the drums but on the guitar (the offbeat upstroke). Those are the basics of ska, so technically, rocksteady and reggae are also ska because they have those basics, but at different tempos and with other things thrown in (like the 'broken up' bass lines in rocksteady and the organ shuffle in reggae).
However, some of the second and third wave 'ska' bands don't always stick to those basics. They usually have the off-beat guitar upstroke but many don't play the bass drum on the third beat of the bar. So they're really playing rock or pop drums with ska guitar, but if people are happy to call that ska, I won't argue with them.