What is Publication?
The Copyright Office defines Publication as:
"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.
Examples include any works distributed to the public through distributors like CD Baby and Distrokid, CDs sold in the community or at church, and sheet music/recordings sold on online stores or personal websites.
For further information see Circular 1, Copyright Basics, section “Publication.”