Transaction T1 updates a row but has not yet committed. Transaction T2 reads this same row's uncommitted new value. T1 then rolls back, meaning T2 has read data that never actually existed in the committed database. What phenomenon does this describe, and at which standard isolation level is it possible?

This scenario is the textbook definition of a dirty read: reading data written by a transaction that has not yet committed (and may never commit, as happens here when T1 rolls back). Dirty reads are possible ONLY at the weakest standard isolation level, READ UNCOMMITTED, which places no restriction on reading uncommitted changes from other transactions. Every stricter isolation level — READ COMMITTED, REPEATABLE READ, and SERIALIZABLE — explicitly prevents dirty reads, though they differ in whether they still permit non-repeatable reads or phantom reads.