A commoner, also known as a common man, common people, subject, or the masses, was someone who was not of nobility, or were an ordinary person who held no title or rank. One who lived under the rule of an empire or other absolute ruler was generally known as a subject, loyal subject, or in antiquity, peasant or peon.
In 2152, Kaitaama facetiously claimed the commoners of Krios Prime had no sense of humor. Trip Tucker, who was stranded with her, later commented he did not think they’d complain if she had died, before characterizing himself as a commoner seconds later. (ENT: "Precious Cargo")
In 2368, Neral said that Pardek's attachment to the common man made “most people” uncomfortable, resulting in his not being invited to official functions for years. (TNG: "Unification II")
Regarding Cardassian jurisprudence and the type of society it had created and preserved, as explained to Miles O'Brien by Conservator Kovat, "on Cardassia, all crimes are solved, all criminals are punished, all endings are happy. Even the poorest of our subjects can walk the streets in the dead of night in perfect safety." (DS9: "Tribunal")
In 2373, Kol bemoaned that he and Arridor would no longer have the unquestioning adulation of the Takarian masses, when it appeared the Ferengi were to be returned to Ferenginar. (VOY: "False Profits")
In 2374, Julian Bashir jokingly claimed that being genetically-engineered did not make it easy to walk amongst the common people. (DS9: "Statistical Probabilities")
In 2381, Beckett Mariner bemoaned being assigned to anomaly consolidation duty, claiming it was when their superiors said, "Hey, peons, come pick up our trash." (LD: "The Spy Humongous")