A scenario involving a series of steps taken by a user while they are interacting with a system.

A user journey is a series of steps (typically 4-12) which represent a scenario in which a user might interact with the thing you are designing. They can be used for 2 main things:

  1. Demonstrating the way users currently interact with the service /
    website / product
  2. Demonstrating the way users could interact with the service / website / product

Reference: http://theuxreview.co.uk/user-journeys-beginners-guide/