a rhythmic unit enclosed between a pair of vertical lines is called?

One of these sites should tell you what you need to know:

http://www.google.com/search?q=reading+music&ie=utf-8&oe=utf-8&aq=t&rls=org.mozilla:en-US:official&client=firefox-a

You will probably find the answer more quickly here:

http://www.soundpiper.com/elements/rhythm.htm

The rhythmic unit between vertical ines is called a "measure". The vertical lines that define a measure are called "bars". Sometimes the measure itself is called a "bar".

The rhythmic unit enclosed between a pair of vertical lines is called a measure or a bar. It is one of the fundamental components of musical notation used to organize music into smaller sections. A measure helps establish the rhythm and meter of a piece of music.

To determine the name of the rhythmic unit enclosed between a pair of vertical lines, you can analyze the structure of the music notation. In sheet music, a vertical line is used to divide the music into measures. Count the number of notes and rests between each pair of vertical lines, and listen to the rhythm of the music to understand where the beats fall. By doing so, you can identify and name the rhythmic units correctly.