building block

noun3

1. •
   A component that is part of a larger construction, such as a brick or a stone.
2. •
   A block made of wood or plastic that is used as a children's toy and can be stacked or joined to form a larger structure.
3. •
   (figurative) A component that is an essential part of a larger system.

   • Words are the building blocks of a language.
   • Cells are the building blocks of life.