Definition from Wiktionary, the free dictionary
- (of separate elements) To join into a single mass or whole.
- The droplets coalesced into a puddle.
- (of a whole or a unit) To form from different pieces or elements.
- The puddle coalesced from the droplets as they ran together.
- (engineering) When two, or more, pieces of metal are bonded together (usually via welding) by liquefying the places where they are to be bonded, coalescing these liquids, and allowing the coalesced liquid to solidify. At the end of this process the two pieces of metal have become one continuous solid.
Related terms 
to join into a single mass
to form from different elements
- second-person singular present active imperative of coalescō