The science of words is closely associated with speech. In tantric literature, all words we express through speech are only at the gross level. The words undergo four levels to express themselves in articulate speech.
First they remain in a causal, undifferentiated, balanced, dormant, and subtlest state. Technically called para, it is in the spinal cord at the level of the navel, manipura.
The next stage is arrived at the middle of the seven chakras called anahata-chakra.
This is the subtler stage called pashyanti, seeing. It lies in the spinal cord behind the heart region.
The next level in the spine is behind the throat at the vishuddha-chakra called madhyama. In that stage the words come to the subtle state, more defining than the earlier one, and ready to become manifest.
At the next stage the words come up to the mouth, become fully manifest with articulation and audibility; this stage is called vaikhari.