Vajra is the fifteenth nityayoga, and its name means the thunderbolt, the diamond-hard weapon of Indra. It is one of the inauspicious yogas, associated with sudden, forceful, and disruptive events, and the panchanga counts it among the configurations to be avoided for auspicious beginnings. Like the others it is found from the combined longitude of Sun and Moon divided into twenty-seven equal parts. The malefic yogas such as Vajra are set aside when an astrologer chooses a muhurta, the tradition reading them as periods of abrupt shock. These are the conventions of the calendar tradition, which fixes the character of each yoga; the astronomical basis remains the arithmetic of the two luminaries' summed longitude through the lunar month.
Vajra, the thunderbolt, is also the diamond in Sanskrit, the same word covering both the weapon of Indra and the hardest known substance, an association the tradition explains by the thunderbolt's supposed indestructibility, and the word gives its name to the Vajrayana, the thunderbolt vehicle, a major branch of Buddhist practice centred on Tibet and the Himalayan region that takes the vajra as one of its central ritual implements and symbols. The nityayoga's own association with sudden, forceful disruption draws on the weapon sense of the word rather than the diamond sense, though both meanings were current together throughout the period the astrological terminology was fixed.