Predictably altering behavior without significantly changing incentives (Universal Principles of Design). Pushing people in a desired direction that will benefit you, the user, or both.
Source Example 1: Images of flies were etched into the urinals of men’s restrooms in an Amsterdam airport to give men a target to aim at. This reduced spillage by 80 percent.
Source: Lidwell, William, Kristina Holden, and Jill Butler. Universal Principles of Design. Gloucester, MA: Rockport, 2003.
Online Example 2: Google Flights takes my previous data and automatically fills in location data, trip length, trip type, etc. to make the initial process of booking a flight easier.
Source: https://google.com/flights
Real Life Example 3: Whole Foods in Oakland tries to reinforce the impact buying organic produce can have on the community. These posters are hanging all over the Whole Foods in my neighborhood and they attempt to inspire people to buy organic produce, therefore spending more money in the store.