In music notation, a tenuto is an articulation mark drawn as a short horizontal line just above the note-head, or just below it if the stem points upward. It indicates that the note should be sustained for its full duration, not clipped or shortened at all with very little pause between it and the following note. It could be considered equivalent to a slur for exactly one note. The word means "held" in Italian.
It looks something like this on the staff (complete with time signature and a quarter note scale):
/\
---| /------------------------------|--------------------||
|/ | | ||
---/--------------------------------|---|----|-----------||
/| 4 | | | | _ * ||
-/-|/\----------------------|----|--|---|----|---*---|---||
| | | 4 | | | | | * | | ||
|--|--|----------|-----|----|----|--|--*--------|----|---||
\ | | | | | * | | | ||
--\|_/-----------|-----|---*--------|-----------|--------||
| | *
\| -*--