This seems like the perfect opportunity for a good quote.
A house divided against itself cannot stand.
That Lincoln was a smart, well versed man. Unity binds together elements and strengthens what it supports. It is so in any aspect of life, even design!
Unity in graphic design is what ties everything together. It is the sense that the design is consistent, it agrees with itself in each element, there is nothing sticking out that says, “I don’t belong!” Sometimes people refer to the principle of unity as harmony.