A nurb spline. 
   Namespace:   Autodesk.Revit.DB  
  Assembly:   RevitAPI  (in RevitAPI.dll) Version: 2015.0.0.0 (2015.0.0.0) 
Syntax
Remarks
 The nurb spline lies in the plane defined by control point, weight,knots,degree. 
 Inheritance Hierarchy
  System Object  
Autodesk.Revit.DB APIObject
Autodesk.Revit.DB GeometryObject
Autodesk.Revit.DB Curve
Autodesk.Revit.DB NurbSpline
 
 Autodesk.Revit.DB APIObject
Autodesk.Revit.DB GeometryObject
Autodesk.Revit.DB Curve
Autodesk.Revit.DB NurbSpline