Unity has two applications. The first relies on rhythm - the repetition of something to create unity. This could be the repetition of an object within the work, such as the number five in this work by Delmuth Charles, or it could be the repetition of an element such as the color red in this work by Pieter Brueghel.
The other way unity is created is through familiarity. When something is seen in a work of art that makes a familiar reference it creates unity. For instance, if a work utilized the American flag in a painting that reference would, at least in part, make a connection giving it unity with previous experiences. Another related concept to unity is continuity.