Ishwari is the term used to refer to the Supreme Mother Goddess. When the term is used independently – Goddesss Ishwari – is The Goddess Shakti (Mother Goddess). When it is used as a suffix – as in Maheshwari, Parameshwari, Vishveshwari etc – it is used to refer to the female energy of the different manifestation of Shiva.
The earliest reference of the use of the term Ishwari is found in the Sri Suktam. Here Shri is referred to as – Ishwari Sarvabhutanam – Mother of all living and nonliving.
Ishwari is also used as suffix to explain the various manifestations of Goddess Shakti as in Goddess Sarveshwari, Goddess Akhileshwari etc.