From my research, I came up with the following translations and conceptualisations for Tantra, which are used also by other authors/seekers in addition to the ones quoted here:
“The term ‘Tantra’ is derived from the Sanskrit root tan for ‘expanding’. Tantra, therefore, means ‘that which expands awareness’.”
“Tan means to expand, while tra means to liberate. So Tantra describes itself as an expansion of the knowledge and practice that liberates us from suffering.”
André van Lysebeth uses the word Tantra “to refer to a body of millennia-old doctrines and, above all, practices” and suggests that one meaning of Tantra is “the instrument to expand the field of ordinary consciousness in order to reach supraconsciousness, the root of one’s being and the wellspring of unknown powers that Tantra seeks to awaken and harness.” 
One of the definitions provided by Georg Feuerstein in “Tantra: the Path to Ecstasy” is that: “…tantra is the ‘expansive’, all-encompassing Reality revealed by wisdom. As such it stands for ‘continuum’, the seamless whole that comprises both transcendence and immanence, Reality and reality, Being and becoming, Consciousness and mental consciousness, Infinity and finitude, Spirit and matter, Transcendence and immanence, or, in Sanskrit terminology, nirvana and samsara.” 
In his list Feuerstein could have included the terms Shiva and Shakti, which are implied in all the pairs he mentions – transcendence (Shiva) and immanence (Shakti), Consciousness (Shiva) and mental consciousness (Shakti), etc.
Mukunda Stiles refers to three categories of Tantra – white, red and pink: “Tantra has two major forms: the primary form is for deepening the connection to your Self (White Tantra) with personal practice. The other forms build on that foundation of self-transformation. […] Red Tantra encourages fulfilment of sexual energies with prolonged intercourse. In contrast, Pink Tantra promotes prolonged energy expression without intercourse. With grounding in the former (White Tantra), the latter (Red or Pink Tantra) becomes more accessible.”
In the last sentence of the above quote, Stiles suggests what I like to emphasis, namely, that sacred sexuality can only be practised in an authentic way once a person has undertaken – and continues to undertake – some serious and dedicated inner work.
According to Osho, Tantra is a practice, not a philosophy or ideology: “…the world of Vigyana Bhairava Tantra is not intellectual, it is not philosophical. Doctrine is meaningless to it. It is concerned with method, with technique – not with principles at all. […] It is not concerned with the ‘why’ of things, it is concerned with ‘how’; not with what is truth, but how the truth can be attained.”
Van Lysebeth appears to agree with Osho’s depiction of Tantra and goes one step further to stress that Tantra is neither a moral code nor a religion: “Tantra is amoral, areligious, atheistic, apolitical, etc. i.e. Tantra espouses, imposes no moral code, no ‘-isms’, it is neither a religion nor a theological doctrine. You don’t become a Tantric ‘convert’, you don’t have to enrol, sign or pledge anything! Tantra judges no one, nothing. It is therefore up to each individual to establish his/her own moral, on the basis of his/her chosen religious beliefs, freely, without preconceptions. Tantra has no organized structure, there is no dogmatic or centralized authority, therefore, no one is empowered to speak on its behalf, not even a guru who at best represents one particular current, one particular Tantric trend and not all of Tantra.”
Excerpt from “Looking for Tantra – living the tantric dream”.
 Dietrich, W. (2012) Interpretations of Peace in History and Culture. Palgrave Macmillan: London & New York, p.34.
 Kempton, S. (2013) Awakening Shakti. Sounds True: Boulder, CO, p.29.
 Van Lysebeth, A. (1995) Tantra – The Cult of the Feminine. Weiser Books: Boston, p.3.
 Feuerstein, G. (1998) Tantra: the Path to Ecstasy. Shamballa Publications: Boston & London, p.2.
 Stiles, M. (2011) Tantra Yoga Secrets. Weiser Books: San Francisco & Newburyport, p.7.
 Bhattacharya, B. (1988) The World of Tantra. Munshiram Manoharlal Publishers Pvt. Ltd: New Delhi, Preface p.xiii.
 Osho. (2010) The Book of the Secrets. St Martin’s Press: New York. p.1.
 Van Lysebeth, A. (1995) Tantra – The Cult of the Feminine. Weiser Books: Boston, p.151.