Threshold
The term "threshold" has a couple of different meanings, but let's start with the simple one. A threshold is the minimum amount of stimulation needed to start a neural impulse (you know, the electrical impulses that travel throughout your body carrying important information). There is a very scientific explanation to this that includes the actual mV (millivolts) of the inside and outside of the neural membrane, how the sodium pottasium channel works, and lots more, but for this definition let's just leave it as is.
For more information on the sodium potassium pump and the neural impulse, consult your book or the biological psychology class notes on AlleyDog.com(http://www.alleydog.com).