what is meant by viscocity of gas or define viscocity of gas?

Viscosity of fluids was answered by previous posts. Viscosity in gases is defined as the "molecular diffusion that transports momentum between layers of flow." (wikipedia). It's just a fancy way of saying that when particles collide, they transfer momentum in such a way that obeys the conservation of momentum law. For an example, lets take the viscosity of air on an airplane wing. The wing is slowed down by this resistance of air molecules to move. Otherwise, the plane could just accelerate towards infinity. Viscosity defined is actually also what helps keep planes afloat. When you blow parallel to the surface of a flat piece of paper, the paper tends to float. Why? One explanation is that the pressure above the paper is now lower than the pressure below the paper; tiny air molecules are repeatedly hitting the paper from below and transferring their momentum onto the paper, which keeps it afloat.
Terminal velocity is also a result of the viscosity of gases. When you drop an object, at some point, the air molecules will resist the movement of the object through air to such an extent that the object will actually maintain a constant velocity (may take a long time to do for a heavy object). This force thus cancels out the gravitational force due to the earth.

viscosity means the tendancy to flow

As JZ quoted from wiki, viscosity of gas is due to momentum exchange between adjacent layers. This is a statistical pheonomenon. It is not a simple conservation of momentum.

Gas particules are constantly/randomly crossing over adjacent layers. if the upper (say) layer initially contains higher average horizontal momentum, then as time goes by, the particules in the upper layer diffuse to the lower layer will increase the average momentum of the lower layer; similarly, particules in the lower layers diffuse into the upper layers will decrease the average horizontal momentum of the upper layers. as a result, the horizontal momentum lost in the upper layers while gained in the lower layer. so horizontal momentum carried by gas particules can diffuse up and down; and its tendency is to homogenize momentum in each layer. The change of momentum in each layer implies it is subject to an internal force from the neighboring layers. In the example described above, the upper layer (rich in momentum initially) is being dragged down from the lower layer (initially poor in momentum), while an equal and opposite force is acted to accel the lower layer. This pair of internal forces are the viscous force internal in the layer. Its magnitude is proportional to the momentum gradient and the rate of particle exchange. The latter is related to the temperature of gas (since more collisions occur in hotter gas). Thus, the coefficient of viscosity (which is actually a measure of the rate of momentum exchange) is higher for higher temperature of gas. For more concise/accurate information, you should look for statistical mechanics.

Caution: The lifting force at a wing foil is due to a different mechanism (B...? sorry forgot the spelling!)

Viscosity is the ease with which a fluid(gas or liquid) flows. Grease has a high viscosity and motor oil a lower viscosity. No rocket science!

in very simple terms viscosity refers to the friction between layers of fluids which cause motion to be retarded.

Vicosity is the resistance to motion in the fluid(gas). It is measured by the coefficient of viscosity. The higher this coefficient the more resistance to movement in the fluid.This coefficient depends on the shape of the moving body in the fluid. Stoke's law is a basic law on the force of viscosity in fluids.