The founders of motivational interviewing, Dr’s. William R. Miller & Stephen Rollnick, have sought to define clearly what MI is, and their descriptions have evolved over time.
The most current definitions are 3:
A layperson´s definition to tell what it is for:
Their current, definitions of Motivational Interviewing are:
Motivational interviewing is a collaborative, person-centered form of guiding to elicit and strengthen motivation for change
A pragmatic practitioners definition to explain why you would use it:
Motivational interviewing is a person-centered counseling method for addressing the common problem of ambivalence about behavior change.
A technical therapeutic definition to explain how it works:
Motivational interviewing is a collaborative, goal-oriented method of communication with particular attention to the language of change. It is intended to strengthen personal motivation for and commitment to a target behavior change by eliciting and exploring an individual’s own arguments for change
Many are familiar with the previous definition of MI:
Motivational interviewing is a directive, client-centered counseling style for eliciting behavior change by helping clients to explore and resolve ambivalence.
You can get details on the background of the new definition in the article: Miller, W.R. & Rollnick, S. (2009) Ten things that Motivational Interviewing is Not Behavioural and Cognitive Psychotherapy, 2009, 37, 129-140.