Catoms, or claytronic atoms, are also referred to as 'programmable matter'. Catoms are described as being similar in nature to a nanomachine, but with greater power and complexity. While microscopic individually, they bond and work together on a larger scale. Catoms can change their density, energy levels, state of being, and other characteristics using thought alone.
A form of catoms were developed on Earth to change telepresence, presumably to enhance it, however, the work was scrapped following the Third World War. In 2168, the crew of the starship Columbia encountered the Caeliar, who were made entirely of catoms. (ST - Destiny novel: Gods of Night)
In 2381, Borg nanoprobes were found to have uncanny similarities to Caeliar catoms, which revealed the connection between the Borg and the Caeliar. When the Caeliar dissolved the Borg Collective, they transformed the nanoprobes of every Borg drone (and former drone) into catoms, restoring their individuality and allowing them to join the Caeliar. (ST - Destiny novel: Lost Souls)