This schema specifies the format for unit definitions. Definitions occur in special-purpose documents containing nothing else, that are made accessible on the internet. BrainML instance documents may refer to these by use of a 'unit' element that is defined in the main brainml schema. They may NOT define units themselves. If a unit needs to be added, a schema authority should submit a unit definitions document to the schema repository. Allowable unit classes, representing the energy or characteristic being measured. Unit class allows applications to specify a restriction on the kinds of units that may occur in a given context. Allowable unit base names. Alternative format using enumerations. Allowable unit prefixes, expressing powers of 10. Single unit definition. This could be used outside of a special-purpose unit-definition document if desired. Name of a unit (composite). Stem of unit name (e.g., "gram", "meter"). Power-of-ten prefix (e.g., "kilo", "centi"). Quantity measured by unit. E.g., "electric-potential". Symbol used for unit. E.g., "kg". Link to corresponding term or definition in an external system. This is the identifier by which the unit will be referenced. Valid characters in identifiers are ONLY letters, numbers, underscore, period, and dash, and an identifier cannot start with a number. The identifier for a domain should be unique within its containing document among both domains and terms. Supports linking of a unit declaration to a corresponding term or definition in an external system. Collection of unit definitions, and valid unit definition document root.