Proportions
The size relationships between a whole and its parts, and between the parts themselves.
In practice
Proportions are size relationships: between a part and the whole, and between parts. They explain why one chair looks elegant and another clumsy even with similar parts, or why a page with wide margins feels calm. Designers use ratios to keep proportions consistent, such as a book page of two to three, or a type scale in which each heading size is a fixed multiple of the body text.
Often confused with Scale
Proportions are internal relationships between parts and whole; scale is size judged against an outside reference, such as a human hand.
Try this
Measure the height and width of three book covers and a phone screen, and compare their ratios.