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• semi
  semi

  noun

  semitrailer.
• semi-
  semi-

  a combining form borrowed from Latin, meaning “half,” freely prefixed to English words of any origin, now sometimes with the senses “partially,” “incompletely,” “somewhat”.
• semi–
  semi–

  A prefix that means “half,” (as in semicircle, half a circle) or “partly, somewhat, less than fully,” (as in semiconscious, partly conscious).

semi

¹ American  

[sem-ee, sem-ahy] / ˈsɛm i, ˈsɛm aɪ /

noun

Informal.

• semis
  plural

1. semitrailer.

2. Often semifinal.

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semi- ² American  

1. a combining form borrowed from Latin, meaning “half,” freely prefixed to English words of any origin, now sometimes with the senses “partially,” “incompletely,” “somewhat”.

   semiautomatic; semidetached; semimonthly; semisophisticated.

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semi ¹ British  

/ ˈsɛmɪ /

noun

1. a semidetached house

2. short for semifinal

3. short for semitrailer

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semi- ² British  

prefix

1. half Compare demi- hemi-

   semicircle

2. partially, partly, not completely, or almost

   semiprofessional

   semifinal

3. occurring twice in a specified period of time

   semiannual

   semiweekly

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semi– Scientific  

1. A prefix that means “half,” (as in semicircle, half a circle) or “partly, somewhat, less than fully,” (as in semiconscious, partly conscious).

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Usage

What does semi- mean? Semi- is a combining form used like a prefix meaning “half.” In some instances, it is used figuratively to mean "partially," "incompletely," or "somewhat." It is often used in a variety of technical and everyday terms. Semi- comes from Latin sēmi-, meaning “half.” The Greek cognate of sēmi- is hēmi-, also meaning “half,” which is the source of English terms such as hemialgia and hemicardia. To learn more, check out our entries for both words.

Commonly Confused

See bi- ¹.

Other Word Forms

Noun Inflected Forms

• semis
  plural

Etymology

Origin of semi¹

By shortening

Origin of semi-²

Middle English < Latin sēmi-; cognate with Old English sōm-, sām- half (modern dial. sam- ), Old High German sāmi-, Sanskrit sāmi-, Greek hēmi-; cf. sesqui-

Example Sentences

See Examples For:

semi semi- semi– semis

On an earnings call, Elon Musk walked back earlier rollout guidance for Tesla’s Robotaxi service, Optimus robots and electric semi truck.

From The Wall Street Journal ● Jul. 23, 2026

Dashcam video of the incident showed traffic slowing down far ahead of the white Jeep as it stopped abruptly in front of the semi, causing the truck to rear-end the vehicle.

From Los Angeles Times ● Jul. 11, 2026

“Some investors may be concluding that Mr. Durn’s departure for a semi company is an indication of that sector having a greater opportunity,” D.A.

From MarketWatch ● Jun. 12, 2026

They now face Bath on Saturday in a straight shoot-out for a home semi after what has been an impressive season.

From BBC ● Jun. 4, 2026

The semi driver said he never saw the bus.

From "A Deadly Wandering: A Mystery, a Landmark Investigation, and the Astonishing Science of Attention in the Digital Age" by Matt Richtel

He “did not adopt a reflexive posture to protect himself and ... he may have fallen in a state of semi- or total unconsciousness,” officials said last month.

From Los Angeles Times ● Nov. 7, 2024

Semi-pedestal sinks are wall-hung sinks with a semi- or half-pedestal attached to the bottom of the sink.

From Seattle Times ● Feb. 28, 2022

The Emirati-Israeli unmanned vessels would be able to operate semi- and fully autonomously and carry out missions including submarine detection and anti-submarine warfare.

From Reuters ● Nov. 18, 2021

He's doubled it in a course of almost eight years, seven-and-a-half years, to be semi- exact.

From Washington Post ● Sep. 26, 2016

We walk over the little semi- lunar bridge Baba has built near a cluster of fig trees; it becomes a great suspension bridge joining cities, and the little pond below, a foamy sea.

From "The Kite Runner" by Khaled Hosseini

Whole vase covered with thin semi– lustrous wash varying from red–brown to black.

From How to Observe in Archaeology by Various

When the semis trade started to wobble late last month, many analysts on Wall Street initially hoped the pain would be short-lived.

From MarketWatch ● Jul. 28, 2026

Doncaster's Greaves came in as the clear favourite but having breezed through her quarter-final, found herself in trouble in the semis.

From BBC ● Jul. 26, 2026

Dyke set the England team two targets: reach the semis of Euro in 2020 and win the World Cup in 2022.

From BBC ● Jul. 13, 2026

The report concludes that ”any pullback in memory semis stocks is likely temporary.”

From MarketWatch ● Jul. 6, 2026

Why would ants choose to live on the side of a highway where they were just going to get run over by cars and semis on a regular basis?

From "Louisiana's Way Home" by Kate DiCamillo