Something fictive is made up, straight from someone's imagination. It sounds a lot like fictitious and fictional, but it has a sharper edge to it: save it for special inventions.
Along with fictitious and fictional, fictive is an adjective related to fiction. They all come from the Latin word fictus meaning "to form," because they have to do with things formed in the imagination. Calling something or someone fictive suggests it has been invented for a special purpose, such as a character in a story that makes a point about society in general. Anthropologists are also fans of the word fictive: if you have an Aunt Mary but she's not technically your aunt, then she's what is known as "fictive kin."