Fantasy names - help!

[Fernando]

Does anyone know of any kind of ereading app that allows me to define my own words? I read a lot of fantasy and sci fi and it's common for me to lose track of which weird character name/species/nation/magical gizmo/planet is which, and I think it'd be really hand to be able to add a note (many allow this, of course) that applies to every instance of the word you applied said note to (none I've found allow this). If anyone knows of something like this I'd love to hear about it, thanks!