Full Idea
I find at least five different senses of "reduction" in the literature - ontological (genes/DNA), property ontological (heat/mean molecular energy), theoretical (gas laws/statistics), logical/definitional (average plumber), and causal (solids/molecules).
Gist of Idea
Reduction can be of things, properties, ideas or causes
Source
John Searle (The Rediscovery of the Mind [1992], Ch. 5.II)
Book Reference
Searle,John R.: 'The Rediscovery of the Mind' [MIT 1999], p.113
A Reaction
A useful pointer towards some much needed clearer thought about reduction. It is necessary to cross reference this list against reductions which are either ontological or epistemological or linguistic.