Symbol which represents a Heroic Spirit. It usually a weapon, but can be a device, armor or even a rare ability.
A Noble Phantasm (宝具 hougu, lit. precious tool) is a weapon or an ability owned by a heroic spirit.
"Noble Phantasm" is the official English name, as given by the katakana reading of its kanji in its encyclopedia entry.
In the legend of a hero, there is not only the man that the hero was, but the anecdotes about him, his arms and weapons; all of this symbolizes his existence. These symbols are the ultimate mysteries that the Servant, the embodiment of the heroic spirit, carries as a trump card. Such is the deadly weapon called "Noble Phantasms". Noble Phantasms are the crystallization of the historical facts and anecdotes that made the heroic spirit famous, but that does not necessarily have to be an offensive weapon; it can be a shield, a ring, a crown, but also a specific ability or a unique method of combat.
The power of the Noble Phantasms are varies. They're usually ranked between E and A class where A being the strongest value. There are certain modifiers and exceptions though. If a rank has a " " value after it it means it's capable of unleashing twice it's supposed power on occasion, there are also " " and probably " " values with a similar meaning. There is also the Ex rank. It's in short means "off the chart". A value soo strong it can't be calculated well. Good example is Ea, Gilgamesh's strongest weapon.
The noble Phantasms also have a certain cathegory: In short what kind of opponent they're good against. Anti-unit Noble Phantasms's abilities are compact and using only a little magical power. Good example is Gae Bolg, a Spear which always strikes the heart. Anti-Army Noble Phantasms are having more power, being able to exterminate whole groups. They're more requiring in magical power though. Gae Bolg's secondary, more devestating ability is of that caliber. Anti-Fortress Noble phantasms are exceedingly powerful but they're wasting trendemous amount of energy. Saber's Excalibur is the best example. There are also Support cathegory Noble Phantasms. They're usually defensive items to protect the Servant.
There's also a special cathegory of Noble Phantasm: Anti-World. So far the only weapon is noted to be of that caliber is Gilgamesh's Ea.
Usually more powerful the Noble Phantasm is the more risque is the use of it. Powerful Noble Phantasms are usually wielded by well known heroes and revealing their abilities usually reveals the Hero himself/herself. Also they're more demanding in both preparation time and magic power usually.