In general, censoring is a term that is used to describe data that is only partially known. The reason it is considered partially known is that all data is constrained between the starting and ending periods of an observational study. This can be information that occurs before a study has begun or after a study has ended. If all the information is contained within the study period itself, the data is not censored. However, this rarely happens.
Censored data can be left or right censored.