In music notation, a trill is a type of ornamentation indicated by the abbreviation "tr." followed by a zig-zag line parallel to the staff beginning directly above the note it affects. When played, a trilled note is rapidly alternated with the note one step higher for its duration.
For instance, a half note with a trill indicated might be played the same as a series of thirty-second notes, and would look something like this:
/\ tr. /\/\/\/
---| /-----------------------------------------------------|
|/ |
---/-------------------------*---*---*---*---*---*---*---*-|
/| 4 O *| *| *| *| *| *| *| *| |
-/-|/\--------|-----------|_|_|_|_|_|_|_|-|_|_|_|_|_|_|_|--|
| | | 4 | |_|_|_|_|_|_|_| |_|_|_|_|_|_|_| |
|--|--|-------|-----------|_|_|_|_|_|_|_|-|_|_|_|_|_|_|_|--|
\ | | | |
--\|_/-----------------------------------------------------|
|
\|
Typically, a trill is begun and ended somewhat slowly and rapidly sped up in the middle for stylistic reasons. This is a normal and expected interpretation of the trill symbol.