What is arrancar?

An Arrancar (アランカル, arankaru) is a Hollow that has removed its mask and gained Shinigami powers. The name "Arrancar" is a Spanish verb meaning "to remove" or "to rip off," a reference to an Arrancar's removal of its Hollow mask. Following the retrieval of the Hogyoku and Aizen's emergence into Hueco Mundo, the Arrancar have become the main antagonists of the series, especially the Espada.