This right relationship -- or synergy -- is a major factor in the whole being "greater than the sum or its parts." For example, water is "more than" mere hydrogen and oxygen, and iron bars are "more strong" welded into a triangle than in any other form.
Wholeness is often conceived as embracing opposites, such as unity and diversity, subjectivity and objectivity, male and female, health and sickness.
This sometimes involves a seeming paradox. How can wholeness mean health on the one hand, and both health and illness on the other? This paradox can be resolved by realizing there are two types of wholeness: existential and preferential.
From the perspective of preferential wholeness, a healthy person is more whole than an ill or injured one, and curing or fixing them is central to their healing. In contrast, from the perspective of existential wholeness, illness and injury are part of the larger wholeness of life, and real healing would entail appreciation and positive engagement with illness and injury as well as wellness. In this view a cancer patient can be healed through deepening insight and engagement with life -- even when cures fail and they die.
Holism is the study and advocacy of wholeness in health, science, politics, or any other area of life.
Wholesome, healthy and holy are all derived from the same root as wholeness.