# Does RP complexity class (usually) mean decidable or recognizable?

Sipser's Introduction to the Theory of Computation defines $P$ complexity class as decided by a deterministic TM in polynomial time, and $NP$ as decided by nondeterministic TM in polynomial time. However:

DEFINITION 10.10:

$RP$ is the class of languages that are recognized by probabilistic polynomial time Turing machines ...

Can someone explain this asymmetry in definition?

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