What's the meaning of tangent in the velocity panel in the particle settings? I wish to know the meaning of the "tangent" option available in particles panel; for a surface in 3D, tangent means a surface. But velocity is a linear, directional entity. 

I am currently working on a project which involves emission of particles along the slope of a cone.
Strictly, 'tangent' means a vector that touches the surface at precisely one point. Not the surface itself. So in the particle setting context:

tangent controls the starting force that is applied to the particle parallel to the emitter surface.

Normal controls the force that is applied to the particle perpendicular to the emitter surface.

To put it another way,

tangent = 1, normal = 0 will make the particle emit 'sideways'.

tangent = 0, normal = 1 will make the particle emit 'out'.

tangent = 1, normal = 1 will make the particle emit at 45degrees to the surface.