What is "sama'i"

Sama'i (also known as usul semai) is a vocal piece of Ottoman Turkish music composed in 6/8 meter. This form and meter ( usul in Turkish) is often confused with the completely different Saz Semaisi, an instrumental form consisting of three to four sections, in 10/8 meter, or usul aksak semai (broken semai in Turkish). Semai is one of the most important forms in Ottoman Turkish Sufi music.