coalescence

from The Century Dictionary.

• noun The act of coalescing or uniting; the state of being intimately joined.
• noun In botany, the organic union of similar parts.
• noun In psychology: the combination of a number of sensible data into a single object-perception.
• noun The unremarked assimilation of a mental content to other mental contents or to conscious dispositions.

from the GNU version of the Collaborative International Dictionary of English.

• noun 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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun The act of coalescing.
• noun phonology The merging of two segments into one resulting from mutual assimilation.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the union of diverse things into one body or form or group; the growing together of parts