Voltage drop is the voltage lost from the battery to the starter. It can be measured by actually measuring the voltage from the hot post on the battery to the starter stud while cranking.

Check the voltage at the battery while cranking. If it is also that low, the battery is weak. If the drop is actually a couple of volts, that's something other than a cable being a size or two too small.

9 is pretty low. I maintain a lot better than that running 2ga. from the back of the car. Check connections and check the drop across the master switch if you have one. Those things don't last forever. Also check starter current draw.