Tantra means system like Gantantra, Partantra, swatantra etc. Swatantra means my own system or rule. Partantra means system of other. If we describe Tan-tra, it means (“Tan – iti- anyatra”) means to achieve Shiv with the help of this body or in other word to search soul through this body.
Body is an instrument to achieve God or Shiva, the science which can relieve the soul from the cycle of Birth, life & death is known as tantraa. Tantra is developed for Jiva/atma to identify his original power & helping him again to become Shiva.
When there was nothing in this world, it was only Brahma existing in the universe. He was colorless, without any shape & size and inactive due to which he was called as “Nirguna”. Brahma or lord Shiva in Santa situation. Later, this shape was changed by his consciousness which is known as “Chiti” or “Shakti” then after realizing himself he entered in “udyota” stage. He further became two and later he divided himself into 36 elements which are basic element of the creation like “Panch Mahabhuta” “Panch tanmatra”, “Panch karmendriya” “Panch gyannendriya” “Antah Karan” “Panch Kanchuka”, “Shudh Tatva”.
He started reflecting his image in the mirror only in this world, this is called “Chiti”. This is clearly mentioned in Kashmir ShaivDarshan by shri Abhinav Gupt.
In my words if we relate this according to science, Brahma, Vishnu & Shiva are same. Brahma is creator, Vishnu is controller & Shiva is destroyer. We can understand it in better way by considering Brahma as oxygen, Vishnu as water & Shiva as Ice. If all ice will melt than there is no life. We know the chemical formula of water is H2O, it means two molecules of Hydrogen and one molecule of Oxygen. We also see the image of Brahma in air, Vishnu in Water & Shiva on Mount Kailash(In Ice).