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]