A sentinel node is the lymph node through which lymph passes from a tumor
back to the lymph channels leading to the heart.  They now inject a tracer
material near the tumor and see which node it goes to, then take just one or
a few nodes from that region.  It is called a sentinel node because it is
the first guard point filtering cancer cells that might escape into the
lymphatics.  The old method took a large number of nodes, which more often
resulted in swelling of the arm, called lymphedema.  The sentinel node
technique is fairly new, being in common use less than ten years.