Polymerase chain reaction (PCR) is a molecular biology technique used to amplify a specific segment of DNA. It involves repeated cycles of heating and cooling to denature, anneal, and extend DNA strands, resulting in the exponential replication of the target DNA sequence. PCR is widely used in various fields such as genetics, forensics, and medical research for applications such as DNA sequencing, genetic testing, and disease diagnosis. It is a powerful tool for studying and analyzing DNA samples with high sensitivity and specificity.