what is non repeatable read

One of the ISO-ANSI SQL defined "phenomena" that can occur with concurrent transactions. If one transaction reads a row, then another transaction updates or deletes the row and commits, the first transaction, on re-read, gets modified data or no data. This is an inconsistency problem within a transaction and addressed by isolation levels.

This is the non repeatable read, mean in a transaction , if your transaction don't get the same result of reads , then it is called as non repeatable read