Would you consider the Spirit World an alternate dimension, or just another way of being in the original dimension? Although I suppose you bring up an excellent point: what is our definition of an alternate dimension? For my purposes, let's refer to an alternate dimension as a self-contained plane of existence that co-exists with one's own. I would also include the notion of parallel dimensions in this; this dimension must be accessible from your home world, and must have some variance to put it at odds with the world you are familiar with.