Parameshwara or Parameshwar means the Supreme God. It is transliterated from the Sanskrit word, ईश्वर  (pronunciation: īśvára). It refers to the sole deity in monotheism. The word "param" meaning the highest is added to Ishwara to intensify the title of God.
Same as Absolute Reality or Absolute Truth (See Para Brahman). The term Parameshwara (Supreme Lord) is used specifically indicate The Supreme Personality of Godhead, who is also the “supreme controller”. And yet it is common that in many Vedic scriptures the term Ishvara is used as a short form for Parameshwara. According to Shaiva puranas and siddhantas, Lord Shiva is Parameshwara. According to Vaishnavite scriptures, Lord Narayana, also known as Vishnu or Hari is Parameshwara. Also note that the term Maheshvara is also used as a synonym to denote Lord Shiva.