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The phrase ordinary language is often used in philosophy and logic to distinguish between ordinary, unsurprising uses of terms and their more specialized uses in theorizing, or jargon. For example, the statements "I find that class of person very annoying" and "Birds fall into a different class from bees" might be said to contain ordinary English uses of class. By contrast, when Bertrand Russell writes, in The Principles of Mathematics, "A class [...] is neither a predicate nor a class-concept, for different predicates and different class-concepts may correspond to the same class." Russell uses the word class in a sense that might or might not correspond neatly to any identifiable ordinary English use of the word; so we might say that he is not using ordinary language, but jargon.
Another example concerns recent use of ‘necessary and sufficient condition’ in logic versus its traditional use in ordinary English. For certain post-WWII logicians, by no means all, necessity and sufficiency are implicational relationships between statements. The assertion that one statement is a necessary and sufficient condition of another means that the former statement is true if and only if the latter is true. That is, the two statements must be either simultaneously true or simultaneously false. In ordinary English, ‘necessary’ and ‘sufficient’ indicate relations between conditions or states of affairs, not statements. Being male sibling is a necessary and sufficient condition for being a brother. Fred’s being male sibling is a necessary and sufficient condition for the truth of the statement that Fred is a brother.
The so-called ordinary language philosophy held that many philosophical problems arose due to confused and inappropriate uses of language that deviated from ordinary language. On their view, philosophers should always attempt to frame their problems in terms of, and to respect the "intuitions" of, ordinary language. Often this point was made by referring to "ordinary English," since the school of philosophy that most vigorously promoted this meta-philosophy was Oxford. This same phrase is still used, occasionally, by (broadly understood) analytic philosophers in supporting or criticizing philosophical positions. Even those who do not hold with the tenets of ordinary language philosophy sometimes regard it a damning criticism of a philosophical view if it involves the use of some term that deviates too widely from ordinary English (ordinary language).
A relatively well understood failure of ordinary language, in terms of its inability to fully describe reality without jargon (in this case mathematics), is quantum mechanics.