In short
Genomic instability is the progressive accumulation of DNA damage — mutations, chromosomal abnormalities, and impaired repair capacity — that occurs over a lifetime of exposure to replication errors, environmental mutagens, and internal stressors like reactive oxygen species.
DNA is under constant assault: replication itself introduces occasional errors, oxidative byproducts of normal metabolism react with DNA bases, and external exposures such as UV radiation and certain chemicals cause additional damage. Cells have an extensive suite of DNA repair pathways — including base excision repair, nucleotide excision repair, and double-strand break repair — but repair is not perfect, and both damage rate and repair efficiency change with age.
This accumulated damage manifests as somatic mutations, chromosomal rearrangements, and copy-number changes, and its rate is not uniform across the genome or across cell types. One especially well-studied consequence is clonal hematopoiesis of indeterminate potential (CHIP), in which a blood stem cell acquires a mutation that gives it a competitive growth advantage, so its mutated descendants gradually make up a growing fraction of blood cells with age — a phenomenon independently associated in large cohort studies with increased cardiovascular disease risk, not just blood cancer risk.
Genomic instability is one of the hallmarks López-Otín and colleagues classified as a primary cause of aging damage (rather than a response to it), because it is generally considered a root driver rather than a downstream consequence of other hallmarks. There is no approved therapy that reverses accumulated genomic instability; prevention (via measures like sun protection and avoiding known mutagens) and monitoring (e.g., for CHIP) are the current, limited practical levers.