Hint:In fluid mechanics the flow of liquid is distinguished on the basis of the velocity of the liquid. The flow of a liquid is categorized into two types on the basis of different parameters such as velocity , one is the Laminar flow and Other one is the turbulent flow.
Complete answer: The flow of the liquid is said to be Laminar when liquid is flowing with a steady flow in the form of multiple layers each having different velocities and the layers move in such a manner that they do not cross or mix with each other which allows it to flow in a steady flow.
The flow of the liquid is said to be Turbulent when liquid is flowing not in a steady flow where the multiple layers do not have a constant velocity it changes continuously and the layers move in a zigzag manner and crosses each other path several times so the path of the particles of the liquid also changes continuously.
Critical velocity is defined as the velocity of the flow of liquid that if the velocity of the flow is less than critical velocity than the flow remains streamlined or laminar and for the flow velocity which is greater than critical velocity the flow becomes turbulent.
Note:Critical velocity is just one of the way to determine the type of liquid flow, Reynolds number which is a numerical value is another way to determine the nature of the flow of the liquid ,generally the reynolds number is preferred over other ways to determine the flow of the liquid.