The Collaborative International Dictionary of English v.0.48:

Semi- \Sem"i-\ [L. semi; akin to Gr. ???, Skr. s[=a]mi-, AS.
   s[=a]m-, and prob. to E. same, from the division into two
   parts of the same size. Cf. Hemi-, Sandelend.]
   A prefix signifying half, and sometimes partly or
   imperfectly; as, semiannual, half yearly; semitransparent,
   imperfectly transparent.
   [1913 Webster]

   Note: The prefix semi is joined to another word either with
         the hyphen or without it. In this book the hyphen is
         omitted except before a capital letter; as, semiacid,
         semiaquatic, semi-Arian, semiaxis, semicalcareous.
         [1913 Webster]

WordNet (r) 3.0 (2006):

semi
    n 1: one of the two competitions in the next to the last round
         of an elimination tournament [syn: semifinal, semi]
    2: a truck consisting of a tractor and trailer together [syn:
       trailer truck, tractor trailer, trucking rig, rig,
       articulated lorry, semi]
    3: a trailer having wheels only in the rear; the front is
       supported by the towing vehicle [syn: semitrailer, semi]

The Jargon File (version 4.4.7, 29 Dec 2003):

semi
 /se'mee/, /se?mi:/

    1. n. Abbreviation for ?semicolon?, when speaking. ?Commands to grind are
    prefixed by semi-semi-star? means that the prefix is ;;*, not 1/4 of a
    star.

    2. A prefix used with words such as ?immediately? as a qualifier. ?When is
    the system coming up?? ?Semi-immediately.? (That is, maybe not for an
    hour.) ?We did consider that possibility semi-seriously.? See also 
    infinite.


The Free On-line Dictionary of Computing (30 December 2018):

semi

   /se'mee/ or /se'mi:/ A spoken abbreviation for semicolon.

   "Commands to grind are prefixed by semi semi star" means
   that the prefix is ";;*", not 1/4 of a star.

   [Jargon File]

   (1995-01-31)