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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  

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 ² British  

/ ˈsɛmɪ /

noun

1. a semidetached house

2. short for semifinal

3. short for semitrailer

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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

It was the first time Forest had reached a European semi final since 1984, while Forest's 5-0 win at Sunderland was their biggest away victory in the top flight since 1995.

From BBC • Jul. 1, 2026

And on its way to the final, the club averaged 2.75 goals per game, even putting five past Bayern Munich in the first leg of their semi.

From The Wall Street Journal • May 30, 2026

Cerebras shares closed 68% above their IPO price on Thursday, reflecting enthusiasm in the market “for a new type of semi company with an impressive product,” D.A.

From MarketWatch • May 15, 2026

Scientists believe the region during the Early Cretaceous was dry to semi dry, conditions that sauropods appear to have favored.

From Science Daily • May 15, 2026

How were we supposed to know that semi was going to skid into a telephone pole?

From "Linked" by Gordon Korman

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