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  

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.

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-; sesqui-

Example Sentences

They were missing leader Marie-Philip Poulin in their opening loss to the U.S., and the U.S. can turn flat, like they did in a sleepy second-period chunk of the semi against Sweden.

From The Wall Street Journal • Feb. 17, 2026

These autonomous or semi autonomous systems could anticipate needs and provide support automatically.

From Science Daily • Feb. 13, 2026

Kodiak AI, a Mountain View-based tech company, is developing autonomous semi trucks it says will improve safety and efficiency on the roads.

From Los Angeles Times • Feb. 2, 2026

A BBC reporter on the scene said two police officers were guarding the door of a semi detached house behind a police cordon.

From BBC • Nov. 13, 2025

Occasionally, she feels the rumble of ground and she’ll look over her shoulder as a semi passes her.

From "A Heart in a Body in the World" by Deb Caletti

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