The meaning of words and phrases matters. Even though common discourse relentlessly erodes the banks of meaning, it is our duty to try to save what is valuable!
The phrase by definition has a precise meaning: the speaker is asserting that a property can be assigned to an object that has been named, by virtue of the fact that the definition of the object requires it to have that property.
People who imitate people who once hung around intelligent people have picked up the verbal tic of snapping the neck of an argument by asserting that something is so "by definition", when it very often is no such thing at all.
By definition, by definition means that the statement that is being made is a rewording or direct consequence of a definition of the thing being discussed.
However, in practice, the phrase by definition is carelessly used to make sloppy connections between ideas, while conveying the impression that this connection is an inherent requirement of the rules of logic and language. Here are some examples:
"What excited Ali was that in the cave were not only pictographs but written inscriptions. Here was evidence that his ancestors not only traded in incense but were literate and civilized. (By definition, a civilization is a society with a written language.)"Nicholas Clapp, "The Road to Ubar".
Civilized: a member of a society with a written language.
Civilization: any society with a written language.
"Part of the reason was quality control: by definition, an Easter egg is an untested feature."Scientific American, November 2011.
Easter Egg: any untested feature in a computer program.
"Though the two men could not coordinate their efforts nor even contact each other, they were witnessing the same disease. By definition", that made it an epidemic.Molly Crosby, "Asleep".
Epidemic: any disease observed by two men.
"By definition, historical sources contradict each other.Constantine Pleshakov.
Historical record: something any two of which contradict each other.
"Each additional home is built, by definition, where there wasn't a home before," Mr Hastings said.The New York Times, 15 September 2007.
Home: something that is built where there isn't one already.
"If you go to the battleground states, the independents, by definition, will pick the winner."Assertive political pundit,
Independent voters: persons whose votes, currently withheld or undisclosed, will decide the presidential election, by virtue of the fact that the blocs already committed to either candidate are insufficient to guarantee election.
"Google, though it contained some of the most highly skilled of humanity, was, like all large software firms, committed to serving mediocrity by definition."David Auerbach, 'A Life in Code',
Large software firm: that which serves mediocrity.
"Almost by definition" he says, "the physical sciences are fields characterized by conceptual elegance and analytic simplicity."M Mitchell Waldrop, "Complexity".
Physical science: any field characterized by conceptual elegance and analytic simplicity.
"Blood is bright red because it is fully oxygenated, yet oxygen is, by definition, colorless."Bill Hayes, "Five Quarts",
Oxygen: an element which has no color.