Parameshvara (epithet)
Parameshvara (, ) is an epithet of the Hindu deity. The term usually  indicates the Supreme being and Supreme Reality in the Hinduism. Vaishnavas consider Lord Vishnu and his avatars such  as Rama and Krishna as Parameshwara, while Shaivas consider Shiva as Parameshwara. Parameshvara is the ultimate and highest reality that eternally pervades all matter for Hindus. He is regarded by devotees to be totality itself, controlling the triple forces of creation, preservation, and destruction. Etymology 
The word is a compound of the Sanskrit words परम meaning 'Supreme' and ईश्वर meaning 'Lord'. Thus Parameshvara literally means 'highest supreme ruler'. Sometimes, other traditions of Hinduism such as Vedanta and Vaishnavism also use the term  as a synonym of Parabrahman within their philosophical perspectives. Vaishnavism
In Vaishnavism traditions, Lord Vishnu is considered as Parameshwara, Maheshwara, and Narayana. Vaishnavas consider Lord Vishnu and his avatars such  as Rama, Krishna etc., as the progenitor of Brahma and Shiva as said in many scriptures such as Bhagavata Purana, Vishnu Purana, Padma Purana, Pancharatra Agamas, Vaikhanasa Agamas and many more. In , a Vaishnava Pancharatra Agama text of Hinduism  Lord Vishnu (Narayana) is revered and worshipped as Parameshwara. Uttaranarayana (a continuation of the Purusha Sukta in the Shukla Yajurveda) also refers to God as Parameshvara with two consorts Sri and Bhu. Bhagavad Gita praises Lord Krishna as Parameshvara (Highest God) in many contexts. In Vishnu Sahasranama, Parameshvara is 377th name of Lord Vishnu. Shaivism

Shaiva Siddhanta accepts the existence of   (three entities),  (the supreme being Paramashiva),  (all atmans) and  (three bondages of Anava, Karma, Maya). As the supreme being, Parameshvara only has the distinct eight characters or predicates which are applied to distinguish him from the other two entities of Shaiva Siddhanta—Pashu and Pasam. They are  (who knows everything),  (with infinite happiness),  (without bondages),  (independent),  (unlimited mercy),  (unrestricted grace),  (wholesome) and  (with pure body). Shaiva Siddhanta states that Parameshvara is in two states—, the form of lord that is moving through 36 tattvas and , the pure form of supreme being beyond everything. These two forms can be compared with the Saguna and Nirguna definitions of Para brahman in the Vedantic tradition. When he is defined with , Paramashiva exists in nine divine forms, Brahma, Vishnu, Rudra, Maheshwara, Sadasiva, Shiva, Shakti, Nadam, and Bindhu in which he is beyond words in his last four formless manifestations known as . The first five are his manifestations with forms and known as . Sadasiva is his mixed form of  and  which is often identified with lingam. Shiva and Shakti exist as inseparable  in the state of  in which they are often identified as the non-dual supreme being Paramashiva and Parashakti. Since they are inseparable and undifferentiated, Shaiva Siddhanta sees them as single oneness, Parameshvara. See also
 Mahadevi
 Mahavishnu
 Parashiva
 Purushottama
 Sadashiva

References

Names of God in Hinduism
Hindu philosophical concepts