Astra

Astra (アストラ) originally referred to the weapons used by the gods of Hindu mythology. However, the Astras in the world of Toaru Majutsu no Index, specifically of the magic cabal Gate of the Gods Come from the Heavens, the term refers to all powers and weapons that use symbols of the god's characteristics.

Principles
The term basically refers to anything magical used by the magic cabal that uses symbols from the gods of the Hindu mythology. The cabal has to downsize the Astras as so humans can wield them, and are nothing but effects. For example, a spear Astra can create wounds with it, but a form of spear cannot be produce itself.

That meant the term Astra could refer to many different things. It would not be mistaken to assume there were different Astras for each element that had been prepared by the gods: fire, water, wind, and earth.

Agni Astra
The Agni Astra (アグニアストラ) is the base form of the Gate of the Gods Come from the Heavens' Agni's Festival Fire. It's legend is derived from the Agni's Aaagneyaastra, a weapon that if discharged would emit flames inextinguishable through normal means.

Agni is associated with fire and lightning, which symbolizes Agni Astra. On one hand, the Astra could be used offensively to destroy the enemies of the god with tremendous lightning and flames. On the other hand, the god that held the Astra held the characteristic of light that possessed symbolic meaning of festival fire. Using this characteristic, the Astra can be used to send out a code in the form of lights that blink at a regular interval, a form of communication based on the length of the flashes.

Shiva Astras
A Shiva Astra (シヴァアストラ) is an Astra which symbolizes the weapons and powers used by, the "Destroyer" in Hinduism, the god of wind, dance, war, and destruction.

Having many different attributes, allows a magician to wield many different Astras based on a single god.

As an Astra of the Gate of the Gods Come from the Heavens, when used by Ureapaddy Exica a trident that glows gold appears in her hands. When she calls out its name, a golden tip that resembled lightning stabs towards its target. It's destructive power is great enough to rip apart and melt the asphalt. As an Astra of the Gate of the Gods Come from the Heavens. Ureapaddy Exica uses it by saying its name, at which point a creaking noise is heard due to the vibration of bones of the target, causing the target to feel great pain. It can be negated by Imagine Breaker by simply touching the body. Wielded by Ureapaddy, it's primary characteristic is how sharp it was It takes form when she calls its name, at which point a heavy weight comes upon her hands but cannot be seen. It can slice through asphalt, to the point that she can walk through the cut it made. As an Astra of the Gate of the Gods Come from the Heavens, who ever wields it is called a Pinakapani (One Who Wields Pinaka), one of the many alternative names for the god Shiva. The Pinaka is an Astra that holds great meaning and its true form is unknown. It can fit in any space provided for it no matter what its form. It is a spiritual item that is that can apparently control all the Astras a wielder has. Ureapaddy uses it to mimic the Hindu gods having more than two arms, allowing her to control her Astras. As an Astra of Gate of the Gods Come from the Heavens, it allows the user to use various spells by expressing them in the form of dance.
 * Torishūra (トリシューラ) is a type of Indian trident. In Hindu mythology it was wielded by Shiva, and is said to have been used to sever the original head of.
 * Pāshupata (パーシュパタ) in Hindu mythology, is said to be a crescent-shaped weapon, irresistible and most destructive personal weapon of Kali and Shiva, discharged by the mind, the eyes, words, or a bow. Never to be used against lesser enemies or by lesser warriors, the Pashupat Astra is capable of destroying creation and vanquishing all beings.
 * Parashu (パラシュ) is an Indian battle-axe. In legends, it was known to have beheaded many an enemy.
 * Pinaka (ピナカ) in Hindu mythology is the divine bow of Lord Shiva gifted to King Janaka by Sage Parashurama for safe-keeping while the sage performed penances.
 * Natarāja (ナタラージャ), The Lord of Dance, in Hindu mythology is another name for Shiva, the cosmic dancer who performs his divine dance to destroy a weary universe and make preparations for the god Brahma to start the process of creation.

Brahma Astras
The Burafumā Asutora (ブラフマーアストラ) is an Astra that symbolizes the bow and arrow weapon used by Brahma, the Hindu god of creation and of the universe. The magic arrow has feathers of wind and an arrowhead of the sun. Once it is fired, it will be automatically guided to its target. It will pierce through any obstacles in the way and surely kill the target. Once the target is dead, it will return to its owner like a boomerang so that the enemy cannot capture it.

The Astra is the ultimate weapon within the Gate of the Gods Come from the Heavens as used by Ureapaddy Exica appears as a large golden bow. It requires at least three shootings stars that are glittering in the sky at once, or at least something represents shooting stars. Due to having an actual physical form, it is the only Astra that is referred to as pure. However, it also can only be used incompletely and taxes the user heavily.

Gandiva
Gāndība (ガーンディーバ) is the bow of Arjuna, the hero of the Hindu epic Mahabhrata, given to him by Brahma.

It is an Astra used by Ureapaddy. When she says its name, she can draw a bluish-white arrow out thin air. When the Brahma Astra bow is aimed towards the sky and the arrow is loosed, a bluish-white light momentarily covers the light, at which point arrows of light rain down upon an area.