There is a difference in meaning between 'tantra' in the historical usage, and the current perverse meaning.
In the period 6th-10th centuries CE, a 'tantra' was a category of spiritual literature, a 'how-to' manual for spiritual enlightenment, in the form of instruction from enlightened Guru to disciple:
The Hevajra Tantra:
The whole world is of the nature of Sahaja – for Sahaja is the quintessence (svarupa) of all; This quintessence is nirvana to those who possess the perfectly pure Citta [mind].
The Kularnava Tantra:
Yoga is not [attained] through the lotus position and not through glancing at the tip of the nose. It is the identity of the self [jiva]and Self [atman] which the yoga experts [visharada] called "Yoga".
For an overview of the early Hindu and Buddhist tantras, see: