Source : Webster's Revised Unabridged Dictionary (1913)

Underlie \Un`der*lie"\, v. t. [AS. underlicgan. See {Under}, and
   {Lie} to be prostrate.]
   1. To lie under; to rest beneath; to be situated under; as, a
      stratum of clay underlies the surface gravel.

   2. To be at the basis of; to form the foundation of; to
      support; as, a doctrine underlying a theory.

   3. To be subject or amenable to. [R.]

            The knight of Ivanhoe . . . underlies the challenge
            of Brian der Bois Guilbert.           --Sir W.
                                                  Scott.

Underlie \Un`der*lie"\, v. i.
   To lie below or under.

Underlie \Un"der*lie`\, n.
   See {Underlay}, n., 1.

Source : WordNet®

underlie
     v 1: be or form the base for
     2: lie underneath
     [also: {underlying}, {underlay}, {underlain}]