Here are excerpts from the glossary of INTERNATIONAL CODE OF ZOOLOGICAL NOMENCLATURE:

monotypy, n.
The situation arising (1) when an author establishes a nominal genus or subgenus for what he or she considers to be a single taxonomic species and denotes that species by an available name (the nominal species so named is the type-species by monotypy) [Art. 68.3]; or (2) when an author bases a nominal species-group taxon on a single specimen but does not explicitly designate it as holotype (holotype by monotypy; see Article 73.1.2).

subsequent monotypy
The situation arising when a nominal genus or subgenus was established before 1931 without any included nominal species, and when only a single taxonomic species denoted by an available name was first subsequently referred to it [Art. 69.3].

synonym, n.
Each of two or more names of the same rank used to denote the same taxonomic taxon.

junior synonym
Of two synonyms: the later established, or in the case of simultaneous establishment that not given precedence under Article 24. See also Article 23.9.

objective synonym
Each of two or more synonyms that denote nominal taxa with the same name-bearing type, or (in the cases of family-group and genus-group taxa) that denote nominal taxa with name-bearing types whose own names are themselves objectively synonymous.

senior synonym
Of two synonyms: the earlier established, or in the case of simultaneous establishment that given precedence under Article 24. See also Article 23.9.

subjective synonym
Each of two or more names whose synonymy is only a matter of individual opinion, i.e. it is not objective. See also Article 61.3.1.