Half; a part.
Latin semi‑, half.
The strict sense of a half occurs only in a minority of words, of which examples are semicircle; semidiameter; semilunar, shaped like a half-moon or crescent; and semiquaver, in British musical terminology a note having the time value of half a quaver, a sixteenth note. A few terms extend the idea to that of occurring twice in some time period, as in semi-annual, occurring twice a year (nominally every half year), and the North American semi-monthly, occurring or published twice a month (or every half month). A related idea occurs in semi-final, a match or round immediately preceding the final, the ‘half-final’.
Most terms in the prefix, however, signal that something is partially or incompletely so: semi-professional, semi-conscious, semi-retired, semi-literate, semi-skilled, semi-derelict, semiprecious, semiconducting (of a substance that has a conductivity between that of an insulator and that of most metals), and semipermeable (of a material or membrane that allows certain substances to pass through it but not others).