# sentinel

/ˈsɛntənəɫ/ (US) · /sˈɛntɪnəl/ (UK)

Source: https://englishreference.com/word/sentinel
Updated: 2026-08-15

## Senses

### 1. a guard

noun · CEFR C1 · Literary

a person, such as a soldier, who stands guard to watch for danger.

For teachers: a person, typically a soldier or guard, stationed to keep watch and alert others of approaching danger.

- The lone sentinel stood at the castle gate throughout the cold night.
- The ancient stone towers served as silent sentinels, overlooking the valley and providing early warning of any advancing armies.

### 2. a medical warning sign

noun · CEFR C2 · Technical

a sign or event that shows a health risk or the start of a disease.

For teachers: an indicator, such as a specific case of disease or an environmental event, that signals a potential public health threat.

- The first case of the flu acted as a sentinel for the coming winter season.
- Public health officials monitor sentinel events in local clinics to detect the early stages of a potential epidemic before it spreads to the general population.

### 3. computing data marker

noun · CEFR C2 · Technical

a special value in a computer program that marks the end of a list of data.

For teachers: a specific value used in computer programming to signal the end of a data structure or to terminate a loop.

- The program looks for a zero as a sentinel to stop reading the list of numbers.
- The algorithm iterates through the array until it encounters the sentinel value, ensuring that the process does not attempt to access memory beyond the allocated buffer.

## Examples

- The sentinel stood quietly at the gate.
- A lone sentinel remained on the castle wall, scanning the horizon for any sign of the approaching army.
- In oncology, the sentinel lymph node is the first one reached by metastasizing cancer cells, serving as a crucial indicator for the extent of the disease's spread.

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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.