The word siteswap may refer to either one of the alternate notations used for juggling patterns, or to any single valid instance of such a notation, or sometimes even, by association, a juggling pattern performed according to such an instance. There exists different variants of siteswap notation for asynchronic, synchronic and multiplexing siteswaps. (See below).
For two excellent resources on this topic, see Allen Knutson's Siteswap FAQ  and Burkard Polster's book The Mathematics of Juggling. Note that these two sources give different definitions for the orbits of the siteswap. See also Polster's Juggling, maths and a beautiful mind.