Nucleus (noun)

1. The central and most important part of an object, movement, or group, forming the basis for its activity and growth.
2. A fundamental, essential, or characteristic part or quality.
3. In biology, the central and most important part of a cell, containing genetic material and controlling cell growth and reproduction.

Origin:

From latin nucleus 'kernel, seed, core'.

Examples:

1. The nucleus of the campaign was the candidate's commitment to social justice and equality.
2. The nucleus of the organization is a dedicated group of volunteers who work tirelessly to advance its mission.
3. The nucleus of the atom consists of protons and neutrons.
4. The cell nucleus is surrounded by a membrane and contains DNA and other genetic material.
5. The nucleus is often referred to as the 'brain' of the cell, since it controls many of its essential functions.