yup it's a little hazy on definitions... in europe, a paludarium is called a riparium, but i disagree.
i always thought of them like this:
terrarium: mimics desert
vivarium: mimics forest (can have water feature, but it is small, usually a pool or stream)
paludarium: mimics riverbank in a forest or desert and has more than 33% water. also has full land section with dirt and such.
riparium: riverbank without actual land. the plants are in planters on the back of the tank and in rafts.
Close! There are certainly some interchangable aspects and some looseness about these terms.
Vivarium is the most inclusive of all the terms. It is any enclosure that has life in it, flora or fauna. So all aquariums, terrariums, paludariums and ripariums are vivariums.
A terrarium is something that is (mostly) land based. Could be desert like with a tarantula, could be like a rain-forest floor. I would argue that if it is mostly land with a small waterfall or a small puddle it is still a terrarium, just one with a water feature.
A paludarium -- I think you got it about right on. Not sure about the exact percent water, but if the water is part is big enough for some fish to swim around and live in it, it qualifies in my mind.
And for a riparium, just think of an aquarium that is only half full, with plants that grow out of the water by design.