Hollow

An arrancar 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. In the manga, arrancar is written using a kanji spelling meaning 'torn mask', but is pronounced as the Spanish 'arrancar'

Few hollows have the potential to remove their masks normally, and even if they manage it the change is not significant. Sōsuke Aizen is able to artificially create arrancar with the Orb of Distortion, which also unlocks much more of their potential than a natural transformation would. The process of turning a hollow into an arrancar is called shinigamification.

Arrancar and ordinary hollows differ in two ways: the location of their holes and the near-total disappearance of their masks. When hollows become arrancar, the chances of them retaining a humanoid form increase with their intelligence; only vasto lorde-class arrancar will definitely assume a humanoid form. If powerful enough, an arrancar can seal his or her excess power into the form of a zanpakutō. Unlike with shinigami, this zanpakutō is simply the hollow's true form sealed into a sword. Therefore, an arrancar's release of their zanpakutō is merely a temporary restoration of their former selves. An arrancar release is called resurrección (Spanish for "resurrection"). Arrancar do not have a known second release.

Those arrancar under Aizen's command also have a standard theme for their clothing. Their uniforms consist of a white jacket, black sash, white hakama, and black socks with white sandals; in essence, the uniforms are basically inverted shinigami uniforms.

Arrancar ranks
The arrancar under Aizen's command are arranged into various ranks based on their strength. Not all of the existing arrancar are under Aizen's command, mostly through lack of interest on Aizen's part.

The Espada (十刃) are the top-ranked arrancar in Aizen's army. The Espada at the time of the group's introduction consists of nine males and one female. Each Espada member is chosen for their superior combat ability and then assigned a number from 1 to 10 that indicates their rank and relative power level, which is tattooed somewhere on their body (Grimmjow's is on his lower back, for instance). The Espada are given control over the Números and are regarded as the strongest arrancar in existence. Similar to the power gap between captains and lieutenants in the Gotei 13, the strength of the Espada far surpasses that of the average Número. As Aaroniero Arruruerie notes, all of the Espada except him were at least adjuchas-class menos before becoming arrancar.

The Números (Spanish: numbers) are the combat specialists. They are assigned a two digit number at birth from 11 to 99 to indicate their age (Número 11 is the oldest, Número 99 is the youngest). The basic Número uniform consists of a white-collared jacket and robes held with a black sash. However, the uniform is customizable, giving each Número a sense of individuality. The fighting ability of Números is thought to be superior to all other arrancar, except for the Espada (current and former).

The Fracción (Spanish: fraction) are a subsection of the Números that directly serve a specific Espada. The number of Fracción for any given Espada varies: Stark and Nnoitra Jiruga would seem to have just one, while Szayel Aporro Granz has at least a dozen.

The Privaron (Spanish: deprived) are former Espada who have officially lost their rank and are assigned a three-digit number. Being former Espada, they are far stronger than the normal Números. They reside in Tres Cifras, which translates literally from Spanish as "three digits". As mentioned by Dordonii, these former "naturally created" Espada were rendered obsolete after Aizen began artificially creating arrancar to meet his own standards.

The Exequias (Spanish: funeral) are arrancar that are part of the execution division, apparently charged with termination of intruders and rebels. They are the equivalent of the Soul Society's Onmitsukidō.