semi

UK: /ˈsɛmi/ US: /ˈsɛmi, ˈsɛmaɪ/ ,(sem′ē, sem′ī)

WordReference Random House Learner's Dictionary of American English © 2025

sem•i /ˈsɛmi, ˈsɛmaɪ/   n. [countable] , pl. -is. 

1. Transport, Informal Terms a semitrailer:
   a jackknifed semi blocking six lanes of traffic.
2. Informal Terms, Often, semis. [plural] a semifinal contest or round.

semi-, prefix. 

1. Pronouns
   • semi- comes from Latin, where it has the meaning "half'':
     semiannual; semicircle.
   • semi- is also used to mean "partially;
     partly;
     somewhat'':
     semiautomatic;
     semidetached;
     semiformal.
   • semi- is also used to mean "happening or occurring twice in (a certain length of time)'':
     semiannual.

WordReference Random House Unabridged Dictionary of American English © 2025

sem•i  (sem′ē, sem′ī), n. [Informal.]

1. Transport semitrailer (def. 1).
2. Often, semis. semifinal (def. 3).

• by shortening

semi-, 

1. Pronouns 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;
   semimonthly;
   semisophisticated.

• Latin sēmi-; cognate with Old English sōm-, sām- half (modern dialect, dialectal sam-), Old High German sāmi-, Sanskrit sāmi-, Greek hēmi-; compare sesqui-
• Middle English

Usage

See bi- ¹.

Collins Concise English Dictionary © HarperCollins Publishers::

semi / ˈsɛmɪ/ noun (semis)

1. a semidetached house
2. short for semifinal
3. 
   short for semitrailer

Collins Concise English Dictionary © HarperCollins Publishers::

semi- prefix

1. half: semicircle
   Compare demi-1, hemi-
2. partially, partly, not completely, or almost: semiprofessional, semifinal
3. occurring twice in a specified period of time: semiannual, semiweekly

Etymology: from Latin; compare Old English sōm-, sām- half, Greek hēmi-