What is publication?

Public distribution.

“Publication has a technical meaning in copyright law. According to the statute, ‘Publication is the distribution of copies or phonorecords of a work to the public by sale or other transfer of ownership, or by rental, lease, or lending. The offering to distribute copies or phonorecords to a group of persons for purposes of further distribution, public performance, or public display constitutes publication. A public performance or display of a work does not of itself constitute publication.’ Generally, publication occurs on the date on which copies of the work are first made available to the public. For further information see Circular 1, Copyright Basics, section “Publication.”
— U.S. Copyright Office