I always read about this term, but what does it actually indicate ? is it measured in Lux or Lx ??
for instance , Sony PD170 has a ML of 1XL , and panasonic DVC30 has 4LUX, what's the dufference ??

Low light performance (minimum illumination) for videocams is expressed in Lux (same as Lx). It defines the amount of light (illumination) which is needed to output a 50% videosignal when a 90% reflective testcard is being shoot. S/N is not involved and makes the subjective acceptability of the (noisy) 50% signal difficult. Better compare cams...
FYI Luminance is different from illumination and stands in video for the B&W part of the signal and in optics for the light generated by a given surface. In display technology it's often called "lightness"