An aryika is a female ascetic in the Digambara tradition, and the separate term is required because the Digambara position does not allow her the same standing it gives the monk.
A different standing
The Digambara tradition holds that the highest ascetic practice includes complete nudity, the sky-clad state from which the tradition takes its name, and that liberation in the present life requires it, and since it does not permit women to practise nudity, it holds that a woman cannot attain the full standing of the highest ascetic or reach liberation from a female body, so the aryika, though a renunciant of great austerity, does not hold the same status as the male muni. The aryika wears a single white garment and keeps a rigorous discipline, but the doctrine assigns her a lower rung than the naked monk.
An old and real division
This is one of the sharpest of the doctrinal differences between the two great branches of Jainism, for the Svetambara tradition holds that women may take the full ascetic life and attain liberation, and indeed that one of the Tirthankaras was a woman, while the Digambara denies both, so the standing of the female ascetic is a real point of division rather than a matter of custom. The aryika thus embodies a specific and consequential doctrinal position, and the need for a separate term marks the Digambara refusal to grant her the standing the Svetambara gives its sadhvis. Within her tradition the aryika lives a life of severe renunciation, honoured as an ascetic, but under a doctrine that sets a limit on what a woman may attain.