Bilwa Ashtakam, also spelled Bilwashtakam, Vilvashtakam, or Bilvashtakam, is the asthakam (ashtak) addressed to Lord Shiva. Bilwa Ashtakam is a highly powerful Sanskrit Shiva stotra which are the eight hymns chanted while offering Bilwa (Vilva) leaves to Lord Shiva. The Bilwa (Aegle Marmelos) leaves are one of the main offering to Lord Shiv and by offering Bilva leaves (Koovalam in Malayalam) a devotee can easily please Lord Shiva.
Bilvashtakam is in Sanskrit. It starts with bowing to God Ganesha. It is a very pious Shiva Stotra. God Shiva likes Bilva (tree) leaf. In this stotra, how Bilva leaf is pious and why it is offered to God Shiva when we worship (pooja) him, is described. While performing Pooja of God Shiva, it is said to be incomplete if we don’t offer Bilva Patra (Leaf).
Chanting Bilwa Ashtakam especially on Maha Shivaratri is considered auspicious. Here is Lyrics of Bilwashtakam with English meaning.