Source : Webster's Revised Unabridged Dictionary (1913)

Underlay \Un`der*lay"\, v. t. [AS. underlecgan. See {Under}, and
   {Lay}, v. t.]
   1. To lay beneath; to put under.

   2. To raise or support by something laid under; as, to
      underlay a cut, plate, or the like, for printing. See
      {Underlay}, n., 2.

   3. To put a tap on (a shoe). [Prov. Eng.]

Underlay \Un`der*lay"\, v. i. (Mining)
   To incline from the vertical; to hade; -- said of a vein,
   fault, or lode.

Underlay \Un"der*lay`\, n.
   1. (Mining) The inclination of a vein, fault, or lode from
      the vertical; a hade; -- called also {underlie}.

   2. (Print.) A thickness of paper, pasteboard, or the like,
      placed under a cut, or stereotype plate, or under type, in
      the from, to bring it, or any part of it, to the proper
      height; also, something placed back of a part of the
      tympan, so as to secure the right impression.

Source : WordNet®

underlay
     See {underlie}
     [also: {underlaid}]

underlay
     n : a pad placed under a carpet [syn: {carpet pad}, {rug pad}, {underlayment}]
     v 1: raise or support (the level of printing) by inserting a
          piece of paper or cardboard under the type; "underlay
          the plate"
     2: put (something) under or beneath; "They underlaid athe
        shingles with roofing paper"
     3: provide with a base, support, lining, or backing; "underlay
        the boards with joists"
     [also: {underlaid}]

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