Specter (noun)
A ghostly apparition, a phantom.
“A specter haunted the cemetery at the old Vasquez manor.”
Specter (noun)
A threatening mental image.
Spectre (noun)
standard spelling of specter|from=British spelling
“The spectre is a ghost of a decapitated young man.”
Wiktionary
Spectre (noun)
a ghost
“a dread of spectres and witches affected every aspect of daily life”
Spectre (noun)
something widely feared as a possible unpleasant or dangerous occurrence
“the spectre of nuclear holocaust”
Oxford Dictionary