Complexity class for concurrent algorithms
Parallel complexity uses the same Landau notation but a different machine model, usually the PRAM which allows polynomially many processors (in the input size). This gives rise to complexity classes like NC.

If you are into algorithms outside of massively-parallel settings, this is all quite useless: the existence of PRAM algorithms does not tell you anything about algorithms on "real" computers, i.e. such with finitely many processors.