building block

1. A block used as a building material, especially a cinder block.
2. A small wooden or plastic cube used as a building toy.
3. A basic element or part of something: Amino acids are the building blocks of proteins.

noun

One of the individual entities contributing to a whole: component, constituent, element, factor, ingredient, integrant, part. See part/whole.

A rectangular masonry unit, other than a brick, made of burnt clay, cement, concrete, glass, gypsum, or any other material suitable for use in building construction.

The noun has 2 meanings:

Meaning #1: a single undivided natural entity occurring in the composition of something else
  Synonym: unit

Meaning #2: a block of material used in construction work