money of account

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A monetary unit in which accounts are kept and that may or may not correspond to actual current denominations.

Wiktionary

1. n. finance A denomination of value used in keeping accounts, for which there may, or may not, be an equivalent coin.

GNU Webster's 1913

1. n. a denomination of value used in keeping accounts, for which there may, or may not, be an equivalent coin; e. g., the mill is a money of account in the United States, but not a coin.