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,
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.