Transition is a term referring to the breakdown of laminar flow, through amplifications of infinitesimal disturbances, to turbulence. A transitional flow may be defined as one that encompasses this process.
The occurrence of physical instabilities that occur in laminar simulations cannot be relied on as an accurate indication of transition. Furthermore, when using turbulence models in low-Reynolds number simulations, the onset of turbulence in viscous layers cannot be predicted with any reliability by the turbulence model itself.
Strictly speaking, STAR-CCM+ has no means of predicting transition. However, a Transition model is available that allows the user to mimic the effect of transition by suppressing the turbulence in a certain pre-defined region.