The spacebar item in The Binding of Isaac that lets Isaac fire tears in a circular pattern is Isaac's Tears, unlocked by defeating the Isaac boss as Isaac.
In the game The Binding of Isaac, the spacebar item that allows Isaac to fire 8 tears in a circular pattern around him is Isaac's Tears. This item is indeed unlocked after defeating the Isaac boss with the character Isaac. It's not to be confused with options A) The Book of Belial, which increases damage, B) Guppy's Head, which spawns attack flies, or D) The D6, which rerolls collectibles.