Nodal line

from The Collaborative International Dictionary of English v.0.48

Nodal \Nod"al\, a.
   Of the nature of, or relating to, a node; as, a nodal point.
   [1913 Webster]

   {Nodal line}, {Nodal point}, in a vibrating plate or cord,
      that line or point which remains at rest while the other
      parts of the body are in a state of vibration.
      [1913 Webster]