The process in which a person's body is reset to (usually) an age of early 20's which takes about 14 months if started when the person is old. Less time can be spent in the rejuvenation tanks but the reduction in age will also be less. Because of the hormones and vitality, sex is almost always at the top of the agenda when people come out of the clinics. It's an expensive process.
The basics of rejuvenation were discovered during the first Martian landings by manipulating the stem cells, and it later grew from there. With rejuvenation humans were able to live for centuries, in different incarnations, and could have different occupations with each rejuvenation. Indeed, rejuvenation caused a single person to marry various times.