ExplanationGradience is a term used to recognise the fact that the boundaries between word classes are not absolutely fixed. Many word classes share semantic and structural characteristics with others, and there is considerable overlap between some of the classes. In other words, the boundaries are "fuzzy", and there are typically ‘better’ members of each category.
For example, let’s look at the fuzziness or gradience of the noun category. So, cat, table and apple are all deemed to be ‘good’ members of category. They are physical objects; they take a possessive form; they can be plural or singular, and they can appear after a determiner.
But what about doorway and sky? These are rather intangible objects, but they are still ‘things’. They are then, slightly ‘less good’ examples of nouns. Year, height and politics are even more marginal examples, because they refer to abstract concepts. Nouns such as swim and run (e.g. I’m going for a swim/run) and swimming (e.g. Swimming is my favourite sport) would more naturally be described as ‘events’ rather than ‘things’ and are rather peripheral examples.