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Isolation level is a characteristic of a transaction and it mandates how one transaction must interact with other transactions accessing the same database, in terms of visibility and locking. There are three levels commonly used: Dirty Read, Read Committed and Repeatable Read. The first is the only mode in Paradox and it's supported by a few RDBMS products. The second is the default for almost any relational engine. The third is the default for Firebird/InterBase^® and it's done without bringing the engine to its knees (not the case with other engines). In addition, InterBase^® offers a higher level, known as Repeatable Read with Stability that can be used for special and short transactions. Thanks to the multi version capability, InterBase^® also allows explicit control over locking management (avoid, wait or give an error) that doesn't have sense on other "classic" engines.

Source: https://www.cvalde.net/document/glossary.htm

See also:
Firebird for the database expert: episode 5 - Locking and record versions
Transaction
TID
Isolation mode

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