  This document describes the current understanding of the IVOA controlled
vocabulary for describing astronomical data quantities, called Unified Content
Descriptors (UCDs).
  The present document defines a new standard (named UCD1+) improving the first
generation of UCDs (hereafter UCD1). The basic idea is to adopt a new syntax
and vocabulary requiring little effort for people to adapt softwares already
using UCD1.
  This document also addresses the questions of maintenance and evolution of
the UCD1+. Examples of use cases within the VO, and tools for using UCD1+ are
also described.
