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I'm told there is such a thing as "water weight" which can explain small fluctuations in weight. However, all mentions of "water weight" I've found have been on dieting websites promoting some miracle weight loss solution that will purge water weight from your body. Given that I'm not in the habit of getting my scientific information from snake-oil salesmen, I'm not sure what the scientific status of the notion is.
What is the scientific status of the notion of "water weight"?
It sounds like what you're talking about is related to water retention, a specialized case of fluid retention, which causes an effect known as edema.
Edema (and thus generalized fluid retention) can be divided into two categories: generalized edema and localized edema. The first occurs all over the body, while the second occurs in only certain parts of the body.
There are various causes of fluid retention. Some include
The specific cause can determine whether the edema is generalized or localized.
Source: The Better Health Channel (approved by the government of the State of Victoria, Australia)
To be even more general, fluid retention is caused by the swelling or increase in pressure of various cavities within the body, including capillaries, the lymphatic system, and the organs I mentioned before. This can eventually cause ruptures to occur, and fluid will leak out. This in turn will cause edema in various parts of the body - again depending on the cause.
"Water weight" appears to be referring to the amount of this excess fluid building up outside of these body cavities. "Water retention" refers to the general phenomenon, while "edema" refers to the associated swelling.
So yes, this is a well-documented phenomenon, although the term "water weight" isn't commonly used.
A final note: As YviDe said, the term may simply be used often in a non-technical way, to refer to some of the weight that is quickly lost in some cases.
