CIMM DEFINITION: The amount of time that a home, Set-Top Box or viewer spends on a piece of content whether viewing or interacting with it in some form.

2: The amount of elapsed time from the initiation of a visit to the last audience activity associated with that visit. Time spent should represent the activity of a single cookied browser or user for a single access session to the web-site or property. (Source: IAB)

Note – According to TIVO, refers to Duration or Dwell Time. TIVO also measures time spent in specific features and in interactive advertising.