Definition of Sushumna:
Sushumna is the main and central nadi or energy channel that joins the Muladhara chakra in a straight line with the Sahasrara passing through the other five chakras with the function of lighting.
Around the Sushumna the left (or lunar) channel and the right (or solar) channel unfold. Of the more than 72,000 nadis, the Sushuma is the main one.