From Halopedia, the Halo wiki
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."