Divine domains were a specific areas of interest for deities, particularly to do with the magic and spells granted to their faithful. Clerics of a particular deity choose two of the deity's domains to focus upon, and gained access to extra spells within those domains. A deity could have as many or as few domains as it pleases, although it was usual to have at least four.
Some domains naturally opposed one another, such as evil and good, or chaos and law. A cleric must conformed to the alignment requirements in order to focus on these domains, even if they belonged to their deity's portfolio.