I am not a psychologist, so my answer is coming from studying burnout and teaching workshops to help people understand it and recover from it. But I was asked this question in a class last week, and wanted to provide a better answer.
Here’s one answer, from Prevention, November, 2015:
“Our evidence is that burnout overlaps depression, that they’re on a continuum, like temperature,” says study coauthor Irvin S. Schonfeld, PhD, a psychology professor at the City College of New York’s Colin Powell School for Civic and Global Leadership. “If you go back to the original paper that was published about burnout, by Herbert Freudenberger—in which he studied people who volunteered at a drug treatment center and who dealt with very difficult patients—one of the ways he described burnout was: ‘It looks like depression.'”
Part of my answer is that burnout, unlike depression, is more situational. This is true or more true with moderate burnout, so if you’re experiencing a difficult situation at work and are able to change that, your burnout symptoms will decrease.
The guru on this topic, Christina Maslach, says in the Prevention article:
“If you’re skeptical that job-related stress and exhaustion could possibly resemble the big black cloud that is depression, you’re not alone. ‘Burnout has always been predictive of depression,’ explains Christina Maslach, PhD, a psychologist at the University of California Berkeley who devised the standard test for burnout, called the Maslach Burnout Inventory, ‘but the measure of burnout used in this study is incomplete.'”
Lastly, remember that according to Christina Maslach’s classic definition of burnout, that it’s lost energy, lost enthusiasm and lost confidence. These three factors combined often create the perfect storm. Most importantly, if you’re suffering from extreme burnout and possibly depression, get help. A professional who understands this complex condition will be an important resource, helping you regain your best self.