As an artist, one of the best ways to make money is to license or assign your work. Although many people use the terms interchangeably, there is a tremendous difference between licensing and assigning your art.
Unfortunately, many companies use terms like "exclusive licenses" for agreements that are actually assignments. Read the fine print carefully, and make sure you know exactly what you are, and aren't, giving away.
A reversion is when something happens that causes the rights to your artwork to spring back to you. Knowing the difference between assignments and licenses, it should be apparent that most reversions should occur in licensing agreements. You grant a company a license to use your artwork for awhile, and then when some predetermined event or events happen, those rights terminate and flow back to you.