hole card

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

• noun A card in poker that is dealt face-down and that the holder is not obliged to reveal before the showdown.
• noun Informal Something held in reserve until it can be used advantageously.

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

• noun poker A playing card, dealt face down, that the holder need not reveal until the showdown
• noun by extension Something held in reserve, or concealed, until it is useful

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

• noun (poker) a playing card dealt face down and not revealed until the showdown
• noun any assets that are concealed until they can be used advantageously