A "non-repeatable read" occurs in database transactions when a transaction reads the same row of data multiple times and gets different values. This inconsistency happens because another transaction modifies the data between the reads. For example, if Transaction A reads a value, and then Transaction B updates that value before Transaction A reads it again, Transaction A will see different results. This phenomenon is a concern in database management systems, particularly in the context of ACID properties, which ensure reliable transactions. To prevent non-repeatable reads, developers can use isolation levels like Serializable or Repeatable Read, which control how transactions interact with each other.