I have been looking at the Navier-Stokes equation, and can't seem to find anywhere a clear description of what velocity it represents.

From what I have read it could be any of the following:

• The 'flow velocity'.
• The velocity of an individual particle (I think this is very unlikely).
• The mean velocity of particles near the point of consideration.

Which of these (if any) is correct and why? (A source would be helpful)