One who accounts or reckons; a person who gives an account or explanation of something.
From account + -er agent suffix, forming a noun for one who performs the action. This archaic term predates the modern 'accountant' and was used from the 14th-16th centuries.
Before 'accountant' became the standard term in the 1700s, English used 'accounter'—a perfect example of how professional titles evolve and older ones disappear even when they're perfectly logical.
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