What is Author?

Legal Definition
An author is narrowly defined as the originator of any written work and can thus also be described as a writer (with any distinction primarily being an implication that an author is a writer of one or more major works, such as books or plays). More broadly defined, an author is "the person who originated or gave existence to anything" and whose authorship determines responsibility for what was created. The more specific phrase published author refers to an author (especially but not necessarily of books) whose work has been independently accepted for publication by a reputable publisher , versus a self-publishing author or an unpublished one .
-- Wikipedia
Legal Definition
In copyright law, an author is a person who creates an original expressive work. The author is also the owner of the copyright unless the author assigns ownership to another in a written agreement. Examples of authors include artists, writers, programmers, choreographers, and translators.

If the work was made for hire, the employer or commissioning party is considered the author of the work.
Legal Definition
One who producos, by his own intellectual labor applied to the materials of his composition, an arrangement or compilation new in itself. Atwill v. Ferrett, 2 Blatchf. 39, Fed. Cas. No. 640; Nottage v. Jackson. 11 Q. B. Div. 637; Lithographic Co., v. Sarony, 111 U. S. 53, 4 Sup. Ct 279, 28 L. Ed. 349.
-- Black's Law Dictionary