Simple Definition:

Pathology is the study of diseases and their effects on the body. Pathologists examine tissues and organs to identify diseases and determine their causes. For example, a pathologist might analyze a biopsy sample [a small piece of tissue taken for analysis] to diagnose cancer.

Very Simple Definition:

Pathology is the study of diseases and how they affect the body. Pathologists are doctors who look at tissues and organs to find diseases.

For example a pathologist can tell if someone has cancer by examining a sample of their biological tissue.