# Mach

/ˈmɑk/ (US) · CEFR C1 · Noun

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

## Definition

a measurement of speed that compares how fast an object is moving to the speed of sound. If a plane is flying at Mach 1, it is traveling at the speed of sound.

For teachers: a dimensionless unit representing the ratio of the speed of an object to the local speed of sound. Typically used in aviation and fluid dynamics to describe supersonic or subsonic velocities.

## Examples

- The jet reached Mach 2 during the test flight.
- Pilots must monitor their instruments closely as the aircraft approaches Mach 1 to manage the effects of the sound barrier.
- Engineers designed the experimental craft to withstand the extreme thermal stresses generated when flying at speeds exceeding Mach 5 in the upper atmosphere.

## Pitfalls

- Not "The plane flew at 2 Machs." but "The plane flew at Mach 2." — Mach is used as a unit prefix followed by a number, rather than a pluralized noun following a number.

## Usage notes

- Usually followed by a number (e.g., 'Mach 1') and functions as a title or proper name in that context; does not take an article when used with a specific value.

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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.