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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.

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.

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

While semi stocks rode the wave of the AI data center investment boom last year, sentiment soured on enterprise software makers like Atlassian, Workday, and Salesforce.

From Barron's • May 22, 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

"Since India hasn't done much in AI or renewable energy or semi conductors, there are not many industries generating the kind of excitement or long-term returns investors now see elsewhere in Asia," says Sengupta.

From BBC • May 14, 2026

The Tesla semi truck offers 40% to 70% fuel cost savings over diesel, benefiting from higher oil prices.

From Barron's • Apr. 30, 2026

When I turn around, he’s gone, and Olga’s friend Angie comes running in, looking like she was the one hit by a semi.

From "I Am Not Your Perfect Mexican Daughter" by Erika L. Sánchez

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