A fictive is a systemmate who believes that they are, in some way, from a work of fiction or media. Fictives can be canon-divergent, meaning that they might not identify with what actually happened in their source, and/or identify their lives as being different to what is actually portrayed in media.
While there is no clear cut line between the various identities, fictives are usually considered different to fictionkin. Many fictives identify their source material as being them in an integral way (which may be due to entering a system with their identity and memories intact), with the entirety of their identity stemming from what happened in their canon or source. Fictionkin usually have more distance between their source and canon due to living longer without complete knowledge of their canon or source, and/or a lack of accompanying memories.
As with other things, there is no clear cut line, and a fictive may identify as fictionkin, and vice versa if the labels fit.