In computer science

Computer science

Computer science or computing science is the study of the theoretical foundations of information and computation and of practical techniques for their implementation and application in computer systems...

, coalescing is the act of merging two adjacent free blocks of memory. When an application frees memory, gaps can fall in the memory segment

Memory segment

x86 memory segmentation refers to the implementation of memory segmentation on the x86 architecture. Memory is divided into portions that may be addressed by a single index register without changing a 16-bit segment selector. In real mode or V86 mode, a segment is always 64 kilobytes in size . In...

 that the application uses. Among other techniques, coalescing is used to reduce external fragmentation

Fragmentation (computer)

In computer storage, fragmentation is a phenomenon in which storage space is used inefficiently, reducing storage capacity and in most cases reducing the performance. The term is also used to denote the wasted space itself....

, 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

Garbage collection (computer science)

In computer science, garbage collection is a form of automatic memory management. The garbage collector, or just collector, attempts to reclaim garbage, or memory occupied by objects that are no longer in use by the program...

.