Common law refers to a body of law that is developed through judicial decisions, rather than being based on written statutes or codes. It originated in England and was later adopted by many countries around the world, including the United States. Common law systems are characterized by their emphasis on precedent - meaning that legal principles are derived from previous court decisions, which serve as a guide for future cases with similar facts. This means that common law is not static, but rather evolves over time as new cases are decided and judges build upon the existing body of law.