Is this question about a word that appears a lot in The Giver or about a word that would apply to the book? I will give one answer for each possibility.
If I could choose a “U” word that really applies to what this book is about, it would be “uniform” or “uniformity.” The society in this book attempts to make its people uniform in almost every way. It does allow for some individual differences in skills and aptitudes when it assigns people their jobs, but it strives for uniformity in practically every other way.
However, the word “uniform” does not show up much in the book. When it does, it refers to clothing, not to the overall society. If I had to pick a “U” word that is in the book, I would pick “units.” I would pick this word because it occurs almost every time the word “family” occurs. I believe that Lowry chooses to call the families in this book “family units” to show how they are not families as we understand them. They are not bonded by blood and not even really bonded by love. Instead, they are just units of people. This word conveys to me the impersonality of their community life.