gml:UnitDefinition 

A gml:UnitDefinition is a general definition of a unit of measure. This generic element is used only for units for which no relationship with other units or units systems is known.
The content model of gml:UnitDefinition adds three additional properties to gml:Definition, gml:quantityType, gml:quantityTypeReference and gml:catalogSymbol.  
The gml:catalogSymbol property optionally gives the short symbol used for this unit. This element is usually used when the relationship of this unit to other units or units systems is unknown.

Element Information

Model

gml:metaDataProperty* , gml:description{0,1} , gml:descriptionReference{0,1} , gml:identifier , gml:name* , gml:remarks{0,1} , gml:quantityType{0,1} , gml:quantityTypeReference{0,1} , gml:catalogSymbol{0,1}
Children: gml:catalogSymbol, gml:description, gml:descriptionReference, gml:identifier, gml:metaDataProperty, gml:name, gml:quantityType, gml:quantityTypeReference, gml:remarks

Attributes

QName Type Fixed Default Use Inheritable Annotation
gml:id ID required 
The attribute gml:id supports provision of a handle for the XML element representing a GML Object. Its use is mandatory for all GML objects. It is of XML type ID, so is constrained to be unique in the XML document within which it occurs.

Used By

Complex Types gco:UnitOfMeasure_PropertyType, gco:UomAngle_PropertyType, gco:UomArea_PropertyType, gco:UomLength_PropertyType, gco:UomScale_PropertyType, gco:UomTime_PropertyType, gco:UomVelocity_PropertyType, gco:UomVolume_PropertyType

Source 

<element name="UnitDefinition" type="gml:UnitDefinitionType" substitutionGroup="gml:Definition">
  <annotation>
    <documentation>A gml:UnitDefinition is a general definition of a unit of measure. This generic element is used only for units for which no relationship with other units or units systems is known. The content model of gml:UnitDefinition adds three additional properties to gml:Definition, gml:quantityType, gml:quantityTypeReference and gml:catalogSymbol. The gml:catalogSymbol property optionally gives the short symbol used for this unit. This element is usually used when the relationship of this unit to other units or units systems is unknown.</documentation>
  </annotation>
</element>

Sample