In linguistics, a certain feature is any qualitative feature that is employed to classify a word or phonemes. These can be either unary or binary in nature, both of which acting as constraints on different forms of linguistics analysis. One example is the distinction between ‘is’ and ‘the’. In English, the word ‘is’ is used as a conjunction while the word ‘the’ is always used as a conjunction. The unary feature ‘is’ does not have a clear distinction from its binary counterpart; however, it is more common to appear as such in languages with grammatical structure that is less structured, in which case the usage may be considered grammatical in nature. A grammatical feature can also be considered a qualitative feature because it can be used to classify an object (a noun) as it were.
An example of a grammatical feature can be illustrated by expanding the above to the case of two words: Jack and Jill. When referring to the pair of Jacks and Jills, we would most likely say ‘Jack Jacks and Jill’, whereas when referring to the word as a whole, we would use the definite article ‘Jill’. It is in this example that two features of language are used, where one is used as a descriptive feature of the object ( Jacks) and the other as a qualitative property of the language (jacks). It can be said that Jack Jacks is a noun while Jill is a noun.
Qualitative features, on the other hand, refer to the independent existence of something with a personification or individuality. For instance, you may say that Jane is a woman. This sentence could well be construed as indicating that Jane is a woman in her specific circumstances (i.e., she is a woman in her employment position). The underlying grammatical concept is the same: she is a woman, in her situation, in employment. It therefore becomes clear that we need to distinguish between descriptive words like ‘a woman’ and the definite article ‘a woman’s’ in order to speak of a particular characteristic of a person.
A characteristic is thus a descriptive word used to describe an existent or particular feature of something. In this sense, it differs from characteristics, which are verbs in tense, and therefore cannot stand for qualities. But the idea that a quality which is not a physical characteristic can be used as a verb makes it much more common in the English language than we might at first imagine.
Take a popular example. Mary is a really attractive girl. A characteristic of this pretty girl is that she has a special attraction for older men. Now, this quality is not physical. It is Mary’s unique ability to have a special attraction to older men.
It does not follow then that Mary has a special quality that Mary has a physical characteristic. We need to differentiate between these to speak of their different characteristics. It would be wrong to say Mary has some physical characteristics while Mary has a quality which is a special quality. So instead of calling Mary a ‘pretty girl’ we could say she has a special beauty which all other pretty girls do not have. That would be more accurate.