ground floor

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

• noun The floor of a building at or nearest ground level.
• noun Informal The beginning of a venture, especially regarded as a position of advantage.

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

• noun The floor of a building closest to ground level; first floor (US)
• noun informal The initial stage of a project

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

• adjective on the floor closest to level with the ground
• noun the floor of a building that is at or nearest to the level of the ground around the building
• noun becoming part of a venture at the beginning (regarded as position of advantage)