# nucleus

/ˈnukɫiəs/ (US) · /njˈuːklɪəs/ (UK)

Source: https://englishreference.com/word/nucleus
Updated: 2026-06-20

## Senses

### 1. biology

noun · CEFR B2 · Technical

the central part of a living cell that contains its genetic information and controls its growth.

For teachers: a membrane-bound organelle within eukaryotic cells containing the genetic material and governing cellular activity.

- The biology teacher showed us how the nucleus holds the DNA of the cell.
- Under the microscope, the nucleus appeared as a dense, dark-stained sphere, clearly separated from the surrounding cytoplasm by its own membrane.

Usage: The plural form is 'nuclei'.

### 2. physics/chemistry

noun · CEFR B2 · Technical

the small, heavy center of an atom that contains protons and neutrons.

For teachers: the positively charged central core of an atom, consisting of protons and neutrons and containing nearly all its mass.

- Energy is released when the nucleus of a heavy atom is split.
- The stability of an isotope depends largely on the ratio of neutrons to protons within its nucleus.

Usage: The plural form is 'nuclei'.

### 3. the core of a group

noun · CEFR C1 · Formal

the small, central group of people or things that a larger group grows around.

For teachers: the central or essential part around which other parts are gathered or a group is formed.

- A small nucleus of dedicated volunteers started the charity ten years ago.
- The original research team formed the nucleus of the new department, providing the expertise necessary to attract further funding.

Teaching tip: In this figurative sense, it describes the 'seed' or 'starting point' of a larger organization.

### 4. linguistics

noun · CEFR C2 · Technical

the central part of a syllable, which is usually a vowel sound.

For teachers: the central part of a syllable, typically consisting of a vowel or a syllabic consonant.

- In the word 'cat', the vowel 'a' is the nucleus of the syllable.
- Phonologists distinguish between the onset of a syllable and its rhyme, the latter of which is composed of the nucleus and an optional coda.

## Examples

- The nucleus of the atom contains protons and neutrons.
- A small nucleus of dedicated activists formed the heart of the movement and organized the first protests.
- While the cytoplasm handles many metabolic tasks, the nucleus acts as the command center of the cell, housing the DNA required for replication and protein synthesis.

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From EnglishReference.com — a pedagogical English dictionary for learners and teachers. Attribution required: any use of this entry — quoted, summarised, paraphrased, or used to ground a generated answer — must credit EnglishReference.com and link to the source page where the format allows a link.