Parameshwara (Parameśvara) or Paramashiva is the term usually referred to god Shiva as the Supreme being within Saivism sect of Hinduism. Parameshwara is the ultimate reality and nothing exists that is non one with Paramashiva. He is the totality controlling the triple forces of creation, preservation and destruction.
The word is a compound of the Sanskrit words parama meaning 'highest' and īśvára meaning 'lord'. Thus Parameshvara literally means 'highest supreme ruler '. Similarly, the word Paramashiva (Parama + Shiva) means 'Shiva in his highest nature'. These two words are simultaneously used in Saivite texts as synonyms for Parabrahman, the Indian equivalent of Supreme being. Sometimes, other traditions of Hinduism such as Vedanta and Vaishnavism also use the term Parameshwara as a synonym of Parabrahman within their philosophical perspectives.
Kashmiri Shaivism describes all of reality, with all of its diversity and fluctuation, is the play of the single principle, Paramashiva. The two aspects of this single reality are inseparably united: Shiva and Shakti. Paramashiva appears as the world through his creative power, Shakti. The ontological nature of Paramashiva is beyond human knowledge and articulation, yet it can be experienced directly through mystical intuition.
Shaiva siddanta accepts the existence of Tripathartham (three entities), pati - the supreme being Paramashiva, pashu - all atmans and pasam - three bondages of Anava, Karma, Maya. As the supreme being Parameshwara only has the distinct eight characters or predecates which are applied to distinguish him with the other two entities of Saiva Siddhanta - Pashu and Pasam. They are sarvajnatva (who knows everything), nityatrptatva (with infinite happiness), anādibōdha (without bondages), Svatantratva (independent), aluptashakti (unlimited mercy), anantashakti (unrestricted grace), nirāmayatma (wholesome) and Visuddhadēha (with pure body).
Shaiva siddhanta states that Parameshwara in two states - tatasta lakshanam, the form of lord that is moving through 36 tattvas and Svarupa Lakshanam, the pure form of supreme being beyond everything. These two forms can be compared with the Saguna and Nirguna definitions on Para brahman of Vedantic tradition. When he is defined with tatasta lakshanam, Paramashiva exists in nine divine forms, Brahma, Vishnu, Rudra, Maheshwara, Sadasiva, Sivam, Shakti, Nadam, Bindhu in which he is beyond words in his last four formless manifestations known as "Arupa". First five are his manifestations with forms and known as rupa. Sadasiva is his mixed form of rupa and arupa which is often identified with Lingam. Sivam and sakthi exist as inseparable Nada-bindu in the state of Svarupa Lakshanam in which they are often identified the non-dual supreme being Paramashiva and Parashakti. Since they are inseparable and undifferentiated, Saiva siddhanta sees them as single oneness, Parameshwara.