TLA stands for the Temporal Logic of Actions, but it has become a shorthand for referring to the TLA+ specification language and the PlusCal algorithm language, together with their associated tools.
TLA+ is based on the idea that the best way to describe things formally is with simple mathematics, and that a specification language should contain as little as possible beyond what is needed to write simple mathematics precisely. TLA+ is especially well suited for writing high-level specifications of concurrent and distributed systems.