The definition I took away from the game was that Alduin was a force of the universe itself.
The 'event' by which the world ends, in the cycle onto which the new world begins.
I suppose in one way, he is the polar opposite at least in the feable minds of mortals, to Akatosh, in that Akatosh is credited with the creation of the world, and Alduin is destined to Swallow it.
Spoiler
Alduin's role as the herrald of the apocalyse isnt negated by his defeat either - His soul is never consumed, and it is said by Para - that The God's may deem for him to return, to continue his work.
From this - I look at Alduin, as a Flood, or Tidal wave with teeth - not necessarilly evil, but not exactly the sort of thing you run around smiling about either.