A resistor installed in series with a load, whose purpose is to limit the current. Current flowing through the resistor will result in a voltage being developed across the resistor. This is the voltage drop. A typical application is to limit current flow in an LED, or to create a voltage drop for a lamp that would be destroyed if exposed to full voltage.
It may also be called a current limiting resistor. As it follows Ohm's Law, the voltage across the series resistor is proportional to the current flowing through it. See the page on LEDs for more information.