In music notation, a mezzo stacatto is an articulation mark drawn as a dot and a short horizontal line just above the note-head, or just below it if the stem points upward, with the dot between the line and the note-head. It indicates that the note should be played more disconnected from the note following than a staccato, but less than a tenuto -- about three-fourths the duration of the beat. It means "half-detached" in Italian.
It looks something like this on the staff (complete with time signature and a quarter note scale):
/\
---| /------------------------------|--------------------||
|/ | | _ ||
---/--------------------------------|---|----|--------.--||
/| 4 | | | | . * ||
-/-|/\----------------------|----|--|---|----|---*---|---||
| | | 4 | | | | | * | | ||
|--|--|----------|-----|----|----|--|--*----.---|----|---||
\ | | | | | * | . | | ||
--\|_/-----------|-----|---*----.---|-----------|--------||
| | * .
\| -*-- .
.