A timestamp is a sequence of characters, denoting the date and/or time at which a certain event occurred. A timestamp is the time at which an event is recorded by a computer, not the time of the event itself. In many cases, the difference may be inconsequential: the time at which an event is recorded by a timestamp (e.g., entered into a log file) should be very, very close to the time of the occurrence of the event recorded.
In some cases, a timestamp can be just the numbering of events, the use of a date_time format to store a timestamp is then not mandatory.
This data is usually presented in a consistent format, allowing for easy comparison of two different records and tracking progress over time; the practice of recording timestamps in a consistent manner along with the actual data is called timestamping.
Timestamps are typically used for logging events, in which case each event in a log is marked with a timestamp. In filesystems, timestamp may mean the stored date/time of creation or modification of a file.