What is the definition of a major species in an aqueous solution and how do we find it?

Generally speaking, you are correct.  The major species in an aqueous solution is the species with the highest concentrations (there can be more than one).

The solubility product often has a great influence on which species are present.  The major species can be either ionic or covalent in nature.  For example, isopropanol is the major species in 70% rubbing alcohol.  Ethanol is the major species in distilled grain alcohol (as high as 95%).  Sodium ions and chloride ions are the major species in 35% brine.

The water as a solvent is often not listed as one of the major species, yet H* and OH- can be listed in acidic or alkaline solutions.