VaguenessType: Informal Fallacy
All things swim in continua.
Vagueness is a characteristic of language, specifically of those terms which classify or qualify objects, that is, common nouns and adjectives. Such terms divide the world of objects into those the term applies tothe extension of the termand those to which it doesn't. For example, the common noun "elephant" divides the world into elephants and non-elephants.
What characterizes a vague term is the existence of borderline cases which do not clearly belong or not belong to its extension. For example, consider the familiar concept of "chair": some things are clearly chairswhat you're sitting on right now, for instanceand others are clearly notfor instance, an elephant even though you might sit upon one. But there are many borderline cases: barstools, beanbag "chairs", school desks, etc.
Vagueness is to be distinguished from ambiguity, though rather fittingly the distinction is vague! An ambiguous term is one with more than one meaning, whereas vagueness is characteristic of a single meaning that has borderline cases. However, it is not unusual for a term to be both ambiguous and vague; in fact, this is the usual case.
Vagueness is a pervasive characteristic of language, and there is no reason to think that it can or should be eliminated. This is because many things in the world that we wish to distinguish lie upon qualitative scales. The color spectrum is a good example of this, and we definitely wish to distinguish colors such as orange and yellow, even though the difference between them is one of wavelength.
Moreover, the fallacy of Vagueness occurs only when the appearance of soundness in an argument depends upon vagueness in its terms. The mere fact of vagueness is not sufficient to justify an accusation of fallacy, but it is sometimes a boobytrap which can cause the unsuspecting person to fall into fallacious reasoning. For this reason, it is useful to be aware of and on our guard against vague terms, so that we can continue to use our vague language without being ensnared by it.
S. Morris Engel. With Good Reason: An Introduction to Informal Fallacies (Fifth Edition) (St. Martin's, 1994), pp. 63-65.