The Sanskrit word Shakta refers to those who worship the Mother Goddess or Divine Shakti. The form this worship takes varies greatly among different traditions and individuals.
In a manner of speaking, Shaktas are often categorized as Dual, Non-Dual, or Mixed: those who meet Shakti in an image or idol (Dual); those who meet Shakti as a radiant consciousness without distinction between inner and outer (Non-Dual); and those who work in both manners (Mixed).
A further, more arcane definition of Shakta is the idea of being a holder of Shakti. That is, a Shakta has or is able to express the Divine Shakti through their own individual being.
Within the International Nath Order, Shakta names the person, not the power. The Uttara Kaula stream that enters the Order was a Shiva–Shakti cult with the stress on the Mother. Mahendranath was clear that a Shakta is not, by that word alone, a Tantrik. Initiation as a Nath or Devi starts the link. It does not, by itself, make the initiate a holder of Shakti. That is the work of sadhana. Do not read Shakta as another word for Shakti. Shakti is the current. Shakta is the one who can carry it.