Noun

(categories)

A group, often named or numbered, to which items are assigned based on similarity or defined criteria.

*

The traditional way of describing the similarities and differences between constituents is to say that they belong to categories'' of various types. Thus, words like ''boy'', ''girl'', ''man'', ''woman'', etc. are traditionally said to belong to the category''' of Nouns, whereas words like ''a'', ''the'', ''this'', and ''that'' are traditionally said to belong to the ' category of Determiners.

This steep and dangerous climb belongs to the most difficult category .

I wouldn't put this book in the same category as the author's first novel.

(mathematics) A collection of objects, together with a transitively closed collection of composable arrows between them, such that every object has an identity arrow, and such that arrow composition is associative.

One well-known category has sets as objects and functions as arrows.

Just as a monoid consists of an underlying set with a binary operation "on top of it" which is closed, associative and with an identity, a category consists of an underlying digraph with an arrow composition operation "on top of it" which is transitively closed, associative, and with an identity at each object. In fact, a category's composition operation, when restricted to a single one of its objects, turns that object's set of arrows (which would all be loops) into a monoid.

Noun

(en noun)

Photograph.

* {{quote-magazine, year=2013, month=July-August, author= Catherine Clabby

, magazine=(American Scientist), title= Focus on Everything , passage=Not long ago, it was difficult to produce photographs of tiny creatures with every part in focus. That’s because the lenses that are excellent at magnifying tiny subjects produce a narrow depth of field. A photo processing technique called focus stacking has changed that.}}