A DNA double-strand break (DSB) is a critical type of DNA damage where both strands of the DNA helix are severed, potentially leading to genomic instability and cell death if not properly repaired. DSBs can occur naturally during cellular processes like replication and recombination, or be induced by external factors such as radiation and certain chemicals, necessitating precise repair mechanisms such as homologous recombination and non-homologous end joining to maintain genomic integrity.