Definition and background:
A thick horizontal line that is used to replace the flags of a series of two or more notes that form a rhythmic grouping. Beamed notes are easier to read than a series of notes with flags. The number of beams used is exactly the same as the number of flags it is replacing. Beams are typically used in rhythmic groupings that span one beat of the specific meter of the composition. This makes it easier to read the rhythms and know what rhythmic patterns are associated with each beat.The beam is typically placed based on the farthest notehead from the middle line of the staff. If the farthest notehead is below the middle line, then the beam will be placed above the staff, and if the farthest notehead is above the middle line, then the beam will be placed below the staff. The shortest stem should be at least one octave long. The beam was first know as a ligature in medieval notation.Also [Eng.] Beam; [Fr.] barre; [Ger.] Balken; [It.] barra; [Sp.] barra. A broad, straight line connecting two or more eighth notes.
A broad, straight line connecting two or more eighth notes
For more information on the music term "Beam" check out these other resources:
Wikipedia - Glossary of Musical Terminology
Virginia Tech Multimedia Music Dictionary
ORB -- Medieval Music Glossary