Given an entity as input, output another entity which is part of the input entity. These are entities of meronym. In linguistics, meronymy is a semantic relation between a meronym denoting a part and a holonym denoting a whole. In simpler terms, a meronym (i.e., output entity) is in a part-of relationship with its holonym (i.e., input entity).

central cavity
marrow
The central cavity is the main cavity of a long bone, in which the marrow is stored. Therefore, marrow is a meronym of central cavity..