A "phantom read" occurs in database systems when a transaction reads a set of rows that match a certain condition, but another transaction inserts or deletes rows that meet the same condition before the first transaction completes. This can lead to inconsistencies, as the first transaction may see different data if it re-reads the same condition later. Phantom reads are a concern in multi-user environments, particularly with the SQL database management system. To prevent phantom reads, developers can use isolation levels like Serializable, which ensures that transactions are executed in a way that avoids these unexpected changes during their execution.