Source : Webster's Revised Unabridged Dictionary (1913)

Underlying \Un`der*ly"ing\, a.
   Lying under or beneath; hence, fundamental; as, the
   underlying strata of a locality; underlying principles.

Source : WordNet®

underlying
     See {underlie}

underlying
     adj 1: in the nature of something though not readily apparent;
            "shortcomings inherent in our approach"; "an
            underlying meaning" [syn: {implicit in(p)}, {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}]

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