The Collaborative International Dictionary of English v.0.48:

Overlap \O"ver*lap`\, n.
   1. The lapping of one thing over another; as, an overlap of
      six inches; an overlap of a slate on a roof.
      [1913 Webster]

   2. (Geol.) An extension of geological beds above and beyond
      others, as in a conformable series of beds, when the upper
      beds extend over a wider space than the lower, either in
      one or in all directions.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Overlap \O`ver*lap"\, v. t.
   1. To lay (one thing) over another; to lap.
      [1913 Webster]

   2. To cover part of.
      [PJC]

The Collaborative International Dictionary of English v.0.48:

Overlap \O`ver*lap"\, v. i.
   1. To be arranged so that a part of one object laying over
      part of another; as, the boards overlapped at their ends.
      [PJC]

   2. To have the ending time of one period or process extend
      beyond the beginning time of another period or process;
      as, the lives of Theodore Roosevelt and Franklin Roosevelt
      overlapped by several decades.
      [PJC]

WordNet (r) 3.0 (2006):

overlap
    n 1: a representation of common ground between theories or
         phenomena; "there was no overlap between their proposals"
         [syn: overlap, convergence, intersection]
    2: the property of partial coincidence in time
    3: a flap that lies over another part; "the lap of the shingles
       should be at least ten inches" [syn: lap, overlap]
    v 1: coincide partially or wholly; "Our vacations overlap"
    2: extend over and cover a part of; "The roofs of the houses
       overlap in this crowded city"