The Collaborative International Dictionary of English v.0.48:

Coalescence \Co`a*les"cence\, n.
   The act or state of growing together, as similar parts; the
   act of uniting by natural affinity or attraction; the state
   of being united; union; concretion.
   [1913 Webster]

WordNet (r) 3.0 (2006):

coalescence
    n 1: the union of diverse things into one body or form or group;
         the growing together of parts [syn: coalescence,
         coalescency, coalition, concretion, conglutination]