Parlance was a way or manner of speaking. The term indicated jargon and phrases associated with a specific group of people.
In 2366, as he had trained for starship command rather than diplomacy, Data expressed his belief that he would not be able to persuade the Human colonists on Tau Cygna V to cede the planet to the Sheliak Corporate, explaining to William T. Riker, "In Human parlance, I do not believe I can get the job done." (TNG: "The Ensigns of Command")