An Astra is a mantra so powerful that can destroy anything it is aimed at. The mantra once chanted anything living or non-living can turn into a supernatural weapon. Often denoted as a sword or bow and arrow, but Astra is much more than that. Any sword having the power of Astra is no ordinary sword.
The bearer of Astra is called Astradhari.
A story from the Ramayana Epic:
Rama and Sita were spending some time together beside a river bank. Rama gets tired and rests on Sita's lap for a while. A demon takes the form of a crow and pecks Sita. She shoos the crow at first, but the demon continues to peck her. Sita restricts all her movements, for her concern is that Rama's sleep may get disturbed. She bears the pain until blood oozes out and falls onto Rama's face. The moment Rama wakes up and looks at Sita bleeding, his rage reaches the sky and he immediately invokes the Brahmaastra on the demon with a straw of grass.