Parasolid is the name of a geometric modelling kernel that was originally developed by a company called « Shape Data Limited », but then it was purchased by Siemens PLM Software.
The PARASOLID CAD format:
This geometric modelling kernel can be licensed by other companies to use its functionalities inside their 3D CAD or CAE software products.
This parasoled kernel will provide some functionalities « ready to use » for 3D geometric modelling such as boolean operations, feature modelling, surfacing, thickening and hollowing, blending and filleting, sheet modeling… etc…
The most common extension you will find for a parasoled file is the extension .x_t (parasoled files exported from CAD have this extension).
There is also another binary format .x_b which is more machine independent and not subject to binary-to-text conversion errors.
If You want to know more about the parasolid CAD format, you can read this article on wikipedia