The phrase by definition means that something is automatically true, either because of the literal meaning of a word or because of the fundamental, inescapable nature of a concept.
While standard dictionaries often treat “by definition” as a simple, single-meaning idiom, in actual usage, speakers deploy it in three distinct ways, ranging from strict logic to rhetorical argument.
The Three Senses of “By Definition”
When evaluating how by definition is used in a sentence, it helps to identify which of the three distinct functions the speaker intends:
1. The Literal (Tautological) Sense
This is the strictest, most literal use of the phrase. It states that a proposition is true automatically because changing it would violate the dictionary definition of the word.
- “A square has four sides, by definition.” (If it doesn’t have four sides, it is not a square.)
- “A bachelor is, by definition, an unmarried man.”
- “A parent is, by definition, older than their biological child.”
- “A century is, by definition, a period of one hundred years.”
2. The Inherent (Axiomatic) Sense
In this sense, the statement isn’t strictly about a dictionary entry, but about an intrinsic, unavoidable feature of the underlying concept.
- “War is, by definition, brutal.” (Brutality isn’t the dictionary definition of “war,” but it is an inherent quality of warfare.)
- “The hospitality industry, by definition, caters to customer comfort.”
3. The Rhetorical (Assertive) Sense
In debate, essays, and political commentary, speakers frequently use by definition as a rhetorical tool. By tacking the phrase onto an opinion or complex argument, the speaker attempts to frame their personal perspective as an undisputed, self-evident truth.
- “Psychology is, by definition, an inexact science.”
- “Any simple solution to this complex crisis is, by definition, wrong.”
- “A system that rewards luck over effort is, by definition, broken.”
Why the “Psychology” Example is Rhetorical
Consider the common phrase: “Psychology is, by definition, an inexact science.”
While many people might view psychology as less rigid than physics or chemistry, “inexactness” is not part of the dictionary definition of psychology. The field spans everything from quantitative neurobiology to qualitative therapy.
By adding “by definition,” the speaker takes a complex, debated topic and attempts to render “inexactness” as the single, absolute core of the discipline. It functions as a rhetorical shortcut, framing a subjective opinion as an undisputed, self-evident truth to prematurely shut down debate.
Editorial Note: In rhetorical usage, saying “X is, by definition, Y” is often a conversational shortcut designed to shut down debate by presenting a debatable stance as an unquestionable fact.
Why Dictionary Definitions Cause Confusion
Search engine snippets and major dictionaries often mix these senses together, creating confusing or repetitive entries:
- Merging Duplicate Meanings: Cambridge Dictionary separates “because of its own features” from “because of the nature of something,” even though both describe the exact same concept.
- Mismatched Examples: Sources like Merriam-Webster and Cambridge Dictionary
frequently group strict mathematical tautologies (“a square has four sides”) right alongside subjective rhetorical arguments (“psychology is an inexact science”), leaving language learners confused about whether the phrase indicates absolute logic or personal stance.
Sentence Examples
“By enrolling in the military, you are, by definition, accepting a high level of personal risk.”
“An economic stimulus package is, by definition, intended as a temporary measure.”
“You cannot call it a unanimous vote if two members dissented. By definition, unanimity requires everyone to agree.”
Similar and Related Expressions
- It Goes Without Saying: It is obvious; it is generally understood or accepted.
- Plain / Clear as Day: Completely obvious; impossible to miss or mistake.
- Ipso Facto: A Latin phrase meaning “by the fact itself” or “as an inevitable result.”
- In and of Itself: Considering something purely by its own intrinsic nature (per se).
- Prima Facie: Based on first impression or self-evident facts.
- Self-Evident: Something that is obvious and clear without needing further proof or explanation.