Censoring occurs when the information on the survival time is incomplete or partially observed. There are different types of censoring in survival analysis:

1. Right-censoring: This is the most common type of censoring in survival analysis. Right-censoring occurs when a participant is still alive or event-free at the end of the study period. In other words, the follow-up time for the participant ends before the event occurs.
2. Left-censoring: This occurs when the true event time is known to be less than a certain time, but the exact time is unknown. For example, if a person is diagnosed with a disease before the study begins, but the date of onset of the disease is not known.
3. Interval-censoring: This occurs when the event time is known to fall within a certain interval, but the exact time of the event is unknown. For example, developing glaucoma in between visits to the optician.

Censoring can have a significant impact on the analysis and interpretation of survival data. It is essential to appropriately handle censoring in survival analysis to obtain accurate estimates of survival times, covariate effects, and other related parameters.