In computer science, coalescing is the act of merging two adjacent free blocks of memory. When an application frees memory, gaps can fall in the memory segment that that application uses. Among other techniques, coalescing is used to reduce external fragmentation, but is not totally effective. Coalescing can be done as soon as blocks are freed, or it can be deferred until some time later (known as deferred coalescing), or it might not be done at all.
Coalescence and related techniques like heap compaction, can be used in garbage collection.