# semaphore

/sˈɛmɐfˌɔː/ (UK)

Source: https://englishreference.com/word/semaphore
Updated: 2026-08-27

## Senses

### 1. visual signalling

noun · CEFR B2

a system for sending messages using two flags or mechanical arms held in different positions to represent letters.

For teachers: a visual signalling apparatus or system, typically involving two flags or mechanical arms, where specific positions correspond to alphabetic or numeric codes.

- The sailor used semaphore to send a message to the other ship across the water.
- Before the advent of radio, naval vessels relied heavily on flag semaphore to communicate tactical instructions across short distances during daylight hours.

### 2. computing

noun · CEFR C1 · Technical

a variable or piece of code that controls access to shared data so that only one process uses it at a time.

For teachers: a synchronization mechanism, often a variable or abstract data type, used to control access to a common resource by multiple processes in a concurrent system.

- The developer added a semaphore to prevent two parts of the app from changing the same file at once.
- The operating system utilizes a binary semaphore to ensure mutual exclusion, preventing race conditions when multiple threads attempt to write to the shared buffer simultaneously.

### 3. to signal nonverbally

verb · CEFR C2 · Literary

to send a message using your arms or body movements instead of speaking.

For teachers: to communicate information through visual gestures or signals, often used figuratively to describe exaggerated nonverbal communication.

- From across the crowded room, she semaphored her boredom by checking her watch repeatedly.
- He stood on the balcony, frantically semaphoring his distress to the rescue team gathered in the valley below.

## Examples

- The sailor used semaphore to send a message to the other ship.
- The operating system uses a semaphore to ensure that only one process writes to the database at a time.
- In concurrent programming, a binary semaphore acts as a mutex, preventing race conditions by forcing threads to wait until a shared resource is released.

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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.