A divine emanation.
From: A Sphere (a frame/limitation of influence, power etc.), Sapphire (Illumination). The 10 divine emanations of Light shine through, spheres/curtains to our world.
Every Sefira is divided internally to ten.
The Jewish Prayer Book.
The word comes from Seder – Order, since the Siddur has the order of the prayers and blessings for the week days, Shabbat and special occasions.
The Prayer Book of the holidays is called Mahzor.