Ambrosia (Greek: ἀΑμβροσία) - Ambrosia is the food of the gods, usually presented in small squares. It is able to cure almost any injury but is unattainable and lethal to mortals. Demigods are advised to use them only in emergencies. An extreme overdose of ambrosia can burn the demigod to ashes.
Backbiter - Backbiter is Luke Castellan's half-celestial bronze, half-tempered steel sword. The celestial bronze part can disintegrate monsters while the tempered steel part can kill mortals. It is also the Scythe of Kronos in a sword form.
Caduceus - Caduceus is the Symbol of Power of the god Hermes. It is short staff entwined with two serpents "George" and "Martha". It can turn into anything that Hermes requires. Its tip can turn something to stone. In Percy Jackson: Sea of Monsters, George and Martha are voiced by Craig Robinson and Octavia Spencer.
Camp Fish-Blood - Camp Fish-Blood is Leo Valdez's nickname for a secret training camp under the Atlantic Ocean. Like Camps Half-Blood and Jupiter, this camp trains heroes who happen to be merpeople (mermaids and mermen) and are run by the Ichthyocentaurs.
Camp Half-Blood - Camp Half-Blood is a secret training facility and sanctuary for young demigods around the world. In the novels, it is located on the northern shore of Long Island Sound in Nassau County, but is in a river valley in the film adaptation of The Lightning Thief. The camp is led by the Greekgod of wine Dionysus (who calls himself "Mr. D" and serves as the Camp Director), and the superlative centaurChiron (the Activities Director and the one who trained Heracles, Achilles, Perseus, etc.). The camp uses the cover name Delphi Strawberry Service, a service selling strawberries in Manhattan and a reference to the Oracle of Delphi.
Camp Jupiter is the Roman equivalent to Camp Half-Blood. Demigods and legacies travel to New Rome after being trained by Lupa, the goddess who trained Romulus and Remus. There is a battle field, the Field of Mars, and a city for veterans of the Legion. The former legionaires live and raise a family. Like Camp Half-Blood, it is protected from attacks. Its entrance is near Caldecott Tunnel and Mount Diablo in San Francisco, California. It is guarded by two members of the Legion. It is led by the two Praetors, who are elected to lead the Legion. Elections are held either on June 24 (Feast of Fortuna) or after victory on the battlefield. They play similar war games like Camp Half-Blood but they are more fierce games, and are a lot harder. When Percy Jackson fought in his first one Hazel referred him to a Demon because she had never seen any one fight like that due to Greeks and Romans having different fighting styles.
Celestial Bronze - Celestial Bronze is a bronze metal formed by the god Hephaestus, and is used as the weapon of choice for Greek heroes. It can kill immortals and monsters, but is harmless to mortals.
Cohorts - Cohorts are used to divide the legionnaires at Camp Jupiter.
Demigod - A demigod is a human/god hybrid who has one immortal (godly) parent and one mortal parent. Most characters in the series, such as Percy Jackson, are demigods.
Denarii - Denarri are ancient Roman currency. In the series, they are used by Camp Jupiter in similar ways as to how the Greeks use golden drachmas.
Determined - Determined is a term describing a demigod who is recognized to be an offspring of a particular god.
Drachmas - Drachmas are ancient Greek currency used for trading. In the series, they are golden; they are used for summoning the Gray Sisters and for Iris messages. The dead are required to use them to pay the ferryman, Charon.
Iris Message - Iris Message is a type of video message that can be used to communicate with others by using a rainbow. The service is managed by Iris: Goddess of Rainbows. To communicate with the other, the caller must first create mist so a rainbow can be formed. The caller holds one golden drachma, clearly recites "Oh Iris, goddess of the Rainbow, accept my offering", throws the drachma into the rainbow, and then states the name of the intended receiver or location. The caller or receiver can end the Iris message by scattering the mist, disrupting the signal.
Imperial Gold - Imperial Gold is a golden metal that is fatal to immortals and half-blood mortals, but it is harmless to humans. It is the weapon of choice for Roman legionaires. The metal was consecrated in ancient times, at the Pantheon in Rome. Its existence was a closely guarded secret of the emperors—a way for their champions to slay monsters that threatened the empire.
Katoptris - Katoptris (which means "mirror") is a ceremonial dagger that was a wedding gift to Helen of Troy and is a sign of power and status. It is currently in possession of Piper McLean. She gets the dagger in The Lost Hero.
Khione - Khione is the princess of ice and snow. She is shown in Lost Hero. She is daughter of Boreas.
Legacy - Legacy is a term used to describe a descendant of a demigod. There are fewer legacies in the series than demigods. Known legacies are Icarus (seen in a dream, grandson of Athena), Octavian (descendant of Apollo), and the Zhang family (descendants of Poseidon).
Mist - The Mist is a type of magic or force that prevents mortals from seeing creatures, places or people from Greek Mythology, such as gods and heroes, and instead they see things that they would find normal. Their line of reasoning gives them explanations for most of the events that occur in the series. As said by Hermes in The Last Olympian, "The Mist shows them anything but the truth."
Mythomagic - A trading card game played by Nico di Angelo (son of Hades) and Frank Zhang (son of Mars).
Mount Olympus - The home of the 12 major Greek Gods. In the Western Civilization, Olympus is currently located far above the Empire State Building on the 600th floor while the original mountain remains in Greece.
Nectar - Nectar (Greek: Νέκταρ) is the liquid form of ambrosia, the food of the gods. Nectar is able to cure almost any injury but it is unattainable and lethal to mortals. Demigods are advised to use them only in emergencies; an extreme overdose of nectar can burn the demigod to ashes.
Oracle - The Oracle is the spirit of the Oracle of Delphi, whose spirit is passed down through a host of humans. After the second world war, Hades cursed the then-current host of the Oracle, denying her the possibility of a successor of the spirit of Delphi. The Oracle, for the most part in the series, was a mummified woman.
Praetors - Praetors are the two descendants of the gods who lead Camp Jupiter. Known praetors are Michael Varus (deceased), Jason Grace (resigned), Reyna Avila Ramírez-Arellano (current), Percy Jackson (briefly), and Frank Zhang (current).
Prophecies - The Prophecies are frequently given by the Oracle in the Percy Jackson and The Lost Hero before quests. In The Son of Neptune and The Mark of Athena, they are given by the harpy Ella. They are generally a verse which gives hints on events that are going to happen in future.
Great Prophecy - The main prophecy in the Percy Jackson & the Olympians series. The prophecy says "A half-blood of the eldest gods shall reach sixteen against all odds; and see the world in endless sleep. A hero's soul cursed blade shall reap. A single choice shall end his days. Olympus to preserve or raze."
Prophecy of Seven - It was used to describe by Rachel Dare (the current host of the Oracle at Delphi) the second Great Prophecy (first mentioned at the end of The Last Olympian). The prophecy says "Seven half-bloods shall answer the call. To storm or fire, the world must fall. An oath to keep with a final breath, and foes bear arms at the Doors of Death."
Riptide - Riptide (also known as Anaklusmos) is a 3-foot-long (0.91 m) magical sword made of celestial bronze, currently in the possession of Percy Jackson. It takes the form of a ballpoint pen (that reads "Anaklusmos" on the side) when not in use. Once the pen is uncapped, it transforms into its functional sword form. The sword is enchanted to reappear in Percy's pocket as a pen if he loses contact with the sword. However, the sword doesn't return if Percy doesn't have pockets. Riptide was forged by Zoë Nightshade and originally took the form of a hair clip. She gave the sword to Hercules as a gift.
Sand Dollar - Sand dollar is a currency given by Poseidon as Percy Jackson's gift on his 15th Birthday.
Seven - The Seven is the term used to describe the Heroes of Olympus, the ones mentioned in the second Great Prophecy (known to Camp Jupiter as the Prophecy of Seven).
Stygian Iron - Stygian Iron is a dark fictional metal which can kill monsters and possibly mortals. When forged, it is cooled in the River Styx, making it indestructible. It is implied that only people of the underworld can wield Stygian Iron weapons. Nico di Angelo is the only one mentioned to wield Stygian Iron.