Mating Dreams are something that can happen while two (or potentially more) Cygnusians are joined together that is essentially a shared dream. The natures of these dreams can vary, as they can be memories as seen in 11x13 "The Furry Inside", or they can be what's tantamount to a shared lucid dream though so far there has been no such mating dream seen. The memories seen in a mating dream can come from either participant, and can even be from either participants' past lives, as seen in the mating dream in 11x13 where we see the past lives of Perry and Vicki: Rael and Melinda, respectively; this is again seen in 11x133c "Free Love and Reincarnation" where we see Vicki, Jock Walsh, and Kacey's previous lives on Chikyu, where it turns out they all knew each other there as well.
Mating dreams seem to be able to be shared even by those preset with the joining Cygnusians, as Vicki was able to share Perry and Christine's mating dreams, even though she wasn't a direct participant herself. This again happens to Vicki when she's present while Christine treats Kacey's neediness though in this situation Vicki was at least having physical contact with Kacey. This situation also brings up an interesting note in that mating dreams seems to be the mind and body's reaction to the joining process, as when Christine was treating Kacey she wasn't actually joining with her, but using her magic powers to simulate the effects of joining on Kacey's body.
Thus far we have only seen two mating dreams: the dream in 11x13, and in 11x133c.