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"

Moby Thesaurus II by Grady Ward, 1.0:

133 Moby Thesaurus words for "overlap":
   accord, accordance, affinity, agree, agreement, answer to,
   applique, arch over, assent, assort with, be consistent, be of one,
   be uniform with, bedizenment, bestraddle, bestride, bridge, check,
   chime, chorus, cohere, coherence, coincide, coincidence,
   compatibility, concert, concord, concordance, concur, conform,
   conform with, conformance, conformation, conformity, congeniality,
   congruence, congruency, congruity, consist with, consistency,
   consonance, consort, cooperate, cooperation, correspond,
   correspondence, dovetail, duplication, duplication of effort,
   embellishment, equivalence, expletive, extend over, extravagance,
   fall in together, fat, featherbedding, filling, fit together, flap,
   fly, frill, frills, frippery, gingerbread, go together, go with,
   hang over, hang together, harmonize, harmony, hit, hold together,
   imbricate, imbrication, interlock, intersect, intersection, jibe,
   jut, lap, lap over, lie over, lock, luxury, match, needlessness,
   oneness, ornamentation, overadornment, overarch, overhang,
   overlapping, overlay, overlayer, overlie, override, padding,
   parallel, parallelism, payroll padding, peace, pleonasm, prolixity,
   rapport, redundance, redundancy, register, register with,
   respond to, ride, self-consistency, shingle, sing in chorus,
   sort with, span, square, square with, stand together, superfluity,
   superfluousness, symmetry, sync, synchronism, tally, tautology,
   timing, uniformity, union, unison, unisonance, unnecessariness,
   verbosity