See CZ:Author for the term as used on Citizendium
An author is the main or final creator of a written work. The term originally had the wider meaning of "one who originates", not restricted to writing, but that sense is not considered here.
In modern creative writing, when a writer conceives and wholly composes an original work of fiction or poetry, then that person can securely be identified as the author, with the work being regarded as a form of property and the author having the benefit of its copyright. However, the idea of a particular combination of words being someone's property is a feature of modern western culture, and the possessive attitude has not always prevailed. Many older works are anonymous, and in Tudor England, for example, composing poetry was considered a proper occupation for gentlemen such as Thomas Wyatt or Walter Ralegh, but they were not always anxious to claim the authorship, with the result that many poems associated with them are of doubtful attribution. In the contemporary world, where writing can produce considerable sums of money, there are occasional claims that certain best-sellers have taken the main ideas and even much of the phrasing from a more obscure author.
The question of authorship can often be complex. For works of the past, common usage normally attributes authorship to the writer who gives a particular form to a work. Thus, for instance, Homer and Malory are considered to be the authors of the Iliad and the Morte d'Arthur, even though it is generally recognised that they have incorporated the work of earlier writers (or in Homer's case, oral tradition), often with little or no modification.
In non-fiction, likewise, authorship is generally attributed to the writer who gives a final form to a mass of information, bringing together material from different sources. If sources are mainly copied, without due acknowledgment, however, the writer is liable to be awarded the title of plagiarist instead.
In non-fiction particularly, but also in fiction, joint authorship is responsible for many works, and it is a prevalent feature of many modern academic publications.