A splatbook is a sourcebook for a particular role-playing game (RPG) that is not needed for play, but is devoted to a particular facet, character class, or fictional faction, providing additional background details and rules options.
For example, a "swords and sorcery" fantasy game might offer splatbooks for each of the races in the setting: humans, dwarves, elves, and others.
[1] Many of these books were titled using similar patterns: clanbooks in Vampire: The Masquerade, tribebooks for Werewolf: The Apocalypse, traditionbooks for Mage: The Ascension, and so forth.
In newsgroups, these were called *books (the asterisk on a computer keyboard being used as a wildcard character).
Shannon Appelcline and Stu Horvath have cited the 1978 book Mercenary, created for the science fiction RPG Traveller, and the 1979 sourcebook Cults of Prax, created for the fantasy RPG RuneQuest, as examples of the splatbook format which preceded its definition.