The term laminar refers to a well-ordered flow, free of macroscopic, non-repeating fluctuations. Laminar flows occur in nature when the Reynolds number (the ratio of viscous to inertial forces) is low enough that transition to turbulence does not occur.
In computational simulations, numerical instabilities can arise from simulating laminar flows at Reynolds numbers that are too large, and these will impede convergence if a steady solution is sought. Therefore, laminar flow simulation is appropriate if it is known a priori that the Reynolds number of the problem is sufficiently low.