building block

Short definition from Wiktionary

Noun

1. 1
   A component that is part of a larger construction, such as a brick or a stone.
2. 2
   A component that is part of a larger construction, such as a brick or a stone: 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. 3
   A component that is part of a larger system.