Ma la is a Chinese phrase that means "spicy-numb," a sensation associated specifically with Szechuan pepper. Szechuan pepper is unrelated to black pepper or chili pepper; in fact, its Chinese name is literally translated as "flower pepper," due to its floral, citrusy quality.
Vast amounts of Szechuan pepper are used in traditional Szechuan dishes to create the taste sensation of ma la, this tingly, burning heat that also numbs the mouth and tongue. It's delicious, and yet most American Chinese restaurants do not use a fraction of the spice needed to achieve this effect; if you ask for a dish "ma la," however, it might get you a little closer!
Some dishes that are often characterized by ma la include ma po dou fu, a very spicy pork and tofu dish, and the mala sauce, which includes sesame oil and other spices in addition to the Szechuan pepper. This is often served with hotpot, a type of stew.
• Washington City Paper article on Szechuan cooking and ma la