Feudalism properly describes a fairly narrow system of government by contract organized such that wealth acrues upward through a series of martial aristocrats in theory to the pinacle where there is a king, but in practice up to the level of least powerful aristocrat which can thwart the attack of his peers. That exact system is pretty rare in my game world, but the mindset behind it is not.
The ancient way of looking at the world is that all property belongs collectively to the government, and that the government - in the form of a sovereign - dispences rights, freedoms, and justice to his subordinates according to their need who in turn in his name dispense a portion of these gifts all the way down to the level of the slaves at the base of the pyramid whose duty it is to work in exchange for the gifts that they recieve. This social system is pervasive throughout my game world regardless of the complexities of the particular government. All property is at some level assumed to be owned by a lord and people use property at his sufferance and can be deprived of it virtually at will. Indeed, those in the a feudal system are generally better off and freer than most peoples of the world because at least they have a contract specifying what their rights in theory are. Areas where there are truly free peoples are fairly rare, especially in human lands were bondage is an accepted way of life, and especially outside of cities.
So when I say that my game world is feudal, this is what I really mean by it - that true private property is rare and that by and large the rights of the people are assumed to be granted by the government rather than the other way around. I don't necessarily mean that a true feudal structure exists, or that the economy is not coin based, or that real power rests in the hands of the great lords, or that society is primarily geared to the production of heavy cavalry. Certainly there are areas of the world - say the King of Morgundmy or Vestland - where all of those things are true, but even in places where that isn't true because there is a strong central monarch or because the noblility is vampires and not human heavy cavalry the general notion everything ultimately belongs to some lord and its by his grace that you are allowed to use and make some profit from it lies behind the social order.
A short run down of the nations of the world is sufficient to show that they aren't all feudal per se:
a) Power is shared between a pair of heriditary kings.
b) Power is held by a series of masked secret societies each of which is responcible for a different aspect of government.
c) Power is held by a council of legal scholars, and new members are voted in to the council by existing ones.
d) Power is held by an autocrat who is elected for life by a peerage of nobles.
e) Power is held by an autocrat who performs a sacred ritual demonstrating that they are the true ruler.
f) Power is held by a number of organized crime families which are headed by vampires.
g) Power is jointly held by the peerage and an assembly of plutocrats who bid for seats on the council.
h) Power is held by a heriditary matriarchy.
i) Power is held by a council of liches.
g) Power is held by an immortal god king.
h) Power is held by the patriarch of the state religion.
i) Power is held by a centuries old archmage.
j) Power is held the heads of several merchant families which have discovered the secret of the potion of longevity, allowing the family heads to live for centuries (if their children don't kill them first).
Nonetheless, I will claim that the above is largely feudal in its political and economic structure, or at least if you think of the high middle ages as feudal. By fuedal, I guess I should say I mean that the world is pre-modern.