Kachū Tenshin Amaguriken (火中天津甘栗拳?), or Chestnuts Roasting on an Open Fire Fist, literally Fire Tenshin chestnut fist, though what is actually supposed to be the translation is Imperial Roasting Chestnuts in the Fire Fist but its Kanji and actual Japanese for it is 火拳でインペリアル焙煎栗, Hi ken de Inperiaru baisen kuri and for the Chinese version the actual Kanji and translation is 栗子焙烧过的一个开放的火拳头, Lìzǐ bèishāoguò de yīgè kāifàng de huǒ quántóu, is Ranma's technique taught to him by Cologne.
The name comes from the method of learning the technique, which eventually trains the user's hand speed such that one can snatch chestnuts from a fire without burning themselves. Because Ranma was affected by the Full-body Cat Tongue pressure point at the time, he could not handle the heat from close proximity to the fire. Thus, he did not master the technique in the conventional manner. Instead, he did so by pulling piranhas from a fish tank using his bare hands. In the manga, this is not treated as a special attack, but as a general speed exercise/enhancement. It is mentioned that due to undergoing it, Ranma can throw several hundred punches so swiftly that it looks like a single strike.
Training vs. Technique
Technically speaking, the Amaguriken is not so much a singular technique as a method of speed and precision training. Whilst it does imbue Ranma with the ability to launch super-swift volleys of punches, this is not treated as a "named technique" in the manga the way that other techniques are.
In the anime, however, Ranma has a tendency to verbally use the name of the training when he makes a dedicated rapid-fire punching attack, and has been shown referring to this attack as "the Chestnut Fist (attack)".