Unity
The degree to which elements read as belonging to one coherent whole rather than as unrelated parts.
In practice
Unity is how strongly the parts of a design seem to belong to one whole. It comes from shared colors, consistent type, aligned edges, repeated shapes and a common theme. A poster in which every element uses the same three colors and two typefaces feels unified; add an unrelated clip-art image and the unity breaks. Unity is not sameness, since strong designs keep variety within a clear family.
Often confused with Balance
Balance concerns how visual weight is distributed; unity concerns whether all the parts read as one coherent family.
Try this
Collect five unrelated images and make them feel like a set by giving them the same color treatment and frame.