P (complexity, $\mathbf{P}$)

$\mathbf{P}$ is the class of decision problems decidable by a deterministic computation in polynomial time.

The complexity class of complements of formal languages in P. A deterministic polynomial-time algorithm deciding a formal language can have its accepting and rejecting outcomes interchanged, so P is closed under complement and equals co-P.

A language in P to which every P language has a logspace many-one reduction. The reduction strength matters: using arbitrary polynomial-time reductions instead would make completeness trivial for nontrivial P decision problems.