The word spectre (or specter) has to do with being haunted — it can be something that literally haunts you, like the ghost of your Aunt Sally who bangs the windows every night. You can also say that a reminder of something painful is a spectre. A spectre can also be a fear of something dreadful happening in the future, like the spectre of a second ice age. Spectre is generally the British spelling, while Americans prefer specter.