The Tesla:

The standard unit of magnetic flux density is the tesla, represented by T. One tesla is equal to one weber per meter squared (1 Wb/m² or Wb . m^-2) whenever the flux is perpendicular to the surface under consideration. At times magnetic flux density is spoken of in terms of the number of "lines of flux" per unit cross-sectional area; this is a vague terminology unless we are told precisely how much magnetic flux is symbolized by a line.