Credit

The credit, abbreviated as "cR," is the standard currency of the Unified Earth Government. As of 2558, the Credit is still in use. Credits can be spent in the form of "credit chips," which seem to be similar to a credit card. Humans sometimes casually refer to credits as "credaroos," analogous to the way 20th and 21st century Americans would refer to dollars as "buckaroos."

A Covenant equivalent currency is gekz.

Halo: Reach
Credits are the in-game currency used to purchase armor permutations in Halo: Reach. Credits are rewarded from gameplay actions, progression in Commendations, and completing challenges. They also function as Experience points by contributing to the player rank.

Halo: Spartan series
Credits are used in Halo: Spartan Assault and Halo: Spartan Strike to purchase additional power weapons, armor abilities and boosters. In Spartan Assault, players can choose to purchase these upgrades by spending XP (earned by playing the game) or by purchasing cR with real-world currency. In Spartan Strike, credits are earned in-game instead of XP, and real-world currency microtransactions are not present.

Halo Infinite
In Halo Infinite, Credits are the in-game currency for purchasing in-game items, including the Battle Pass, consumables like the XP Boost and XP Grant, armor customization, armor coatings, and weapon coatings. These are purchased with real-world currancy.

Trivia

 * Despite the universal usage of credits as currency throughout the UEG, "dollars" have occasionally been mentioned in reference to the standing unit of currency in the Halo universe. In the original edition of Halo: The Fall of Reach, it is mentioned that "trillions of dollars" have been spent in the research of Covenant culture, although this is changed to "credits" in the 2010 reissue. In Halo: Glasslands, Admiral Parangosky states that Dr. Halsey escaped with "billions of dollars' worth of UNSC resources". In the trailer of Halo 3: ODST, there are monitors that are intended to announce information to citizens. One of the messages says "Your tax dollars at work." However, this can be understood as an idiomatic usage of the iconic phrase as opposed to a literal reference to the functioning currency.
 * In Halo: The Fall of Reach, when meeting John-117 for the first time and showing him an antique US quarter, Dr. Halsey states that "People used coins like this for currency a long time ago, when Earth was the only planet we lived on," suggesting that there are no coins or papers used in the 26th century. However, in the audio logs in Halo 3: ODST, an old woman is heard breaking an ATM with money in the form of coins and presumably paper coming out. Another ATM, found in gameplay, which also houses an audio log, is also constantly spitting out money until the Audio Log is collected. It is unknown whether this is an oversight or if the currency is different than credits. However, in light of the information in ODST, Halsey's comment could be interpreted as referring to the specific type of coin alone, as opposed to physical currency altogether.
 * If a Marine is killed with friendly fire in Halo 3, another one may say "Okay, now you owe me twenty credits!"
 * The headhunters Jonah-B283 and Roland-B210 would constantly bet twenty credits while on missions.
 * The Warthog trailer for Forza Motorsport 4 lists the price of the 2554 M12 Warthog as 343,000 cR, which serves as a reference to 343 Industries rather than a canon price. Note that Forza also uses cR as currency to purchase vehicles, but does not serve as a reference to Halo.