building block

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

• noun A block used as a building material, especially a cinder block.
• noun A small wooden or plastic cube used as a building toy.
• noun A basic element or part of something.

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

• noun A component that is part of a larger construction.
• noun A block made of wood or plastic that is used as a children's toy.

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

• noun a block of material used in construction work
• noun a single undivided natural thing occurring in the composition of something else