Partial loss of, partial gain of, or movement within large structures of DNA.
Chromosomal rearrangements include deletions, duplications, inversions, translocations, and reciprocal translocations. Insertions also might be included in this group, though not necessarily representing, in terms of chromosomal rearrangements, something other than, for example, tranlocations (i.e., inserted DNA has got to come from somewhere).
As with mutations in general, these rearrangements typically are maladaptive though occasionally can be adaptive. In addition, though usually pictured in terms of large quantities of DNA making up potentially a substantial fraction of a chromosome, in fact, all of the above-listed processes can occur on smaller scales, even in some cases down to individual nucleotides (e.g., insertions and deletions).
Many of these processes can occur as errors in association with molecular recombination, though smaller errors, particularly small deletions and insertions, instead can be replication errors. In any case, chromosomal rearrangements can be viewed essentially as mutations as they represent replicable changes in nucleic acid sequence.
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