Made by more than one hand, with each hand still visible. Of work that belongs to a group without belonging to nobody.
From collaborate (Latin collaborare, com- + laborare, to work together); see collaboration.
Visibility is the test that separates this from mere delegation — a thing made collaboratively still shows the seams where one hand's work meets another's, and smoothing those seams away until no hand can be told from any other would turn collaboration into something else entirely.