CIMM DEFINITION: Total count of unique Viewing Devices which initiated at least one session for a particular VOD asset or set of VOD assets during a given reporting period. Today, unique viewing devices refer to set-tops. A Unique Viewing Device is counted each time a specific Viewing Device (as designated by a Viewing Device Identifier) initiates a VOD session and is counted once and only once during the reporting period. Unique Device or Unique Device counts are specific to the reporting period (month, week, day, etc.). At the beginning of a new reporting period, the counts of Unique Devices are reset. (Source: CTAM Advanced Cable Solutions Consortium, DAI Metrics)