Other forms: ulterior motives
An ulterior motive is a hidden reason for doing something. Someone who offered you a ride to work just so they could recite their newest poem to you and ask what you think of it has an ulterior motive.
The word ulterior isn't too common outside of the phrase ulterior motive. It comes from a Latin word meaning "more distant," so you could think of an ulterior motive as a reason for doing something that lies beyond the first viewing. Someone with an ulterior motive has plans that aren't first apparent, and that's influencing their choice to involve themselves in something.