Fragmentation is the degradation in allocation behavior and available allocation space that is caused by lots of small heap "holes" where objects have been previously garbage collected. If the heap is full of holes and all of them are fairly small, it might be the case that there is no place to put a freshly allocated larger object, even if the total amount of free space is a significant part of the total heap size. Fragmentation is one of the hardest problems to solve concurrently and efficiently in modern garbage collectors. It is always addressed with some kind of compaction.

See also Compaction.