BOB

The BOB is a special type of enemy encountered in Halo: Reach. The BOBs usually take the form of yellow/gold-armored Sangheili Rangers. In the ONI: Sword Base level, BOBs take the form of a white-armored Elite with randomly generated armor, ranging from Minor to Field Marshall. These white-armored BOBs always dual-wield Plasma Rifles, regardless of rank.

Description
Killing one of BOB's will reward the player the "Seek and Destroy" medal. However, the medal can only be viewed on Bungie.net. In the post-game carnage report, the BOB is referred to as an "Elite light vehicle". On Bungie.net, their description is simply "Wipe them out."

The name "BOB" is a reference to Bungie's Marathon video game series. In the series, all of the encountered human characters, besides the player, are known as "BOBs". "BOB" is an acronym for "Born on Board", a reference to the characters having been born onboard the titular colony ship. The AI Leela instructed the player to save as many as possible, but Bungie encourages the player on secret terminals to kill them all. The Vidmaster's Challenge states that the player must "Never, ever leave a single BOB alive."

BOBs are considered easter eggs, and thus have no canonical explanation in the Halo story.

Trivia

 * Each BOB has a set time limit before it vanishes. For some BOBs, this time frame begins as soon as it is alerted to the player's presence. For others, the countdown begins once the player enters a certain area, thus it is possible for the player to miss the BOB entirely if it is not spotted. Also, if you assassinate a BOB, it might disappear. If it does, you'll be assassinating air.
 * In Halo:Reach all ranks of elites can be BOB's.
 * In the level ONI: Sword Base, there are many more BOB's than in other levels.