The tantric path is also known as Vajrayana. Why is it called Vajrayana?

In the Buddhist scriptures, especially in tantra, the vajra symbolizes oneness; all things are one and the same, inseparable. What are one and the same?

In tantra, ground and fruition are one and the same; ultimate truth and relative truth are one and the same; clear light and emptiness are one and the same.

This is not the case in sutra, where ground and fruition are clearly separate.

In sutra, ground is the illusory world we see in front of us, which includes sentient beings, the external world of mountains, rivers, earth, etc., and emptiness; fruition is the attainment of the three resultant bodies of buddhahood—the dharmakaya, sambhogakaya, and nirmanakaya. We ordinary people are not yet buddha, but through practice we can attain buddhahood after three asamkhyeya kalpas. Sentient being is the cause; buddhahood is the result.

~Depicted from GATEWAY TO VAJRAYANA PATH - Vajrayana Terminology