Error 'Division undefined System Code External entry point'

When saving or activating a record in Salesforce, you may encounter the following error message or a variation of it:

CANNOT_INSERT_UPDATE_ACTIVATE_ENTITY. The formula in the "SomeWorkflowOrValidationOrFormulaField" rule or process is invalid due to the following: Division undefined: (System Code) External entry point

What this error means:
Despite the word "Division" in the error, this error is not related to the Salesforce Divisions feature (used to segment data by business unit). Instead, this error indicates that a formula field involved in a Workflow Rule, Validation Rule, or Formula Field is dividing a number by zero because the "Blank Field Handling" setting is configured to "Treat blank fields as zeroes."

Example scenario: A formula field is used as the criteria for a Workflow Rule and specifies FieldA / FieldB. If the value calculated for FieldB equals 0 at runtime (because it is blank and blank fields are treated as zeroes), Salesforce throws the "Division undefined" error. This can occur on any object where formula fields perform division operations without accounting for blank or zero denominators.