[1] : The Collaborative International Dictionary of English v.0.48

underlying \un`derly"ing\, a.
   1. lying under or beneath; as, the underlying strata of a
      locality.
      1913 webster

   2. hence: fundamental; basic; as, underlying principles;
      underlying causes.
      1913 webster +pjc

   3. implicit; not immediately obvious; requiring careful
      scrutiny to discover; as, the underlying sarcasm in her
      seemingly innocuous remark..
      pjc

[2] : WordNet (r) 2.0

underlying
     see underlie
see also:
underlie 

[3] : WordNet (r) 2.0

underlie
     v 1: be or form the base for
     2: lie underneath
     also: underlying, underlay, underlain
see also:
underlying underlay underlain 

[4] : WordNet (r) 2.0

underlying
     adj 1: in the nature of something though not readily apparent;
            "shortcomings inherent in our approach"; "an
            underlying meaning" syn: implicit inp, inherent
     2: located beneath or below
     3: being or involving basic facts or principles; "the
        fundamental laws of the universe"; "a fundamental
        incompatibility between them"; "these rudimentary truths";
        "underlying principles" syn: fundamental, rudimentary
see also:
implicit inp inherent fundamental rudimentary 

[5] : Moby Thesaurus II by Grady Ward, 1.0

53 moby thesaurus words for "underlying":
   ab ovo, basal, basic, basilar, between the lines, bottom, cardinal,
   constituent, constitutive, covert, critical, crucial, cryptic,
   delitescent, dormant, elemental, elementary, esoteric, essential,
   foundational, fundamental, gut, hibernating, hidden, indispensable,
   latent, lurking, material, muffled, mystic, necessary, needful,
   obfuscated, obscured, occult, of the essence, original, possible,
   potential, primal, primary, primitive, radical, rudimentary,
   sleeping, submerged, substantial, substantive, under the surface,
   unmanifested, veiled, virtual, vital

Underlying >> U Words
Underlying, definition of term: Underlying
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