evening star

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

• noun A planet, especially Venus or Mercury, that is prominent near the western horizon shortly after sunset.

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

• noun The planet Venus as seen in the western sky in the evening.
• noun Less commonly, the planet Mercury as seen in the western sky in the evening.
• noun A common evening primrose, Oenothera biennis.

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

• noun a planet (usually Venus) seen at sunset in the western sky