Building-block (or module)
DEFINITION 1: A unit of construction or composition; especially: something essential on which a larger entity is based.
SOURCE: Merriam Webster dictionary.
DEFINITION 2: An independent unit that can be combined with others and rearranged, replaced, or interchanged to form different structures or systems.
SOURCE: Gentile, D., Peter (2013). Theory of Modularity, a Hypothesis. Published by Elsevier B.V.