spook

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Informal A ghost; a specter.
• noun Slang A secret agent; a spy.
• noun Offensive Slang Used as a disparaging term for a black person.
• intransitive verb To haunt.
• intransitive verb To startle and cause nervous activity in; frighten.
• intransitive verb To become frightened and nervous.

from The Century Dictionary.

• To play the spook.
• noun A ghost; a hobgoblin.

from the GNU version of the Collaborative International Dictionary of English.

• noun A spirit; a ghost; an apparition; a hobgoblin.
• noun (Zoöl.) The chimæra.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A spirit returning to haunt a place.
• noun A ghost or an apparition.
• noun A hobgoblin.
• noun espionage A spy.
• noun A scare or fright.
• noun dated, pejorative A black person.
• verb To scare or frighten.
• verb To startle or frighten an animal

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• verb frighten or scare, and often provoke into a violent action
• noun someone unpleasantly strange or eccentric
• noun a mental representation of some haunting experience