Definition
The timestamp in a block header is a 4-byte Unix timestamp representing the approximate time the block was mined. It does not need to be perfectly accurate; the protocol allows some flexibility (must be greater than the median of the previous 11 blocks and less than two hours in the future).
Timestamps are used to calculate elapsed time between difficulty adjustment periods. Miners can also vary the timestamp slightly as an additional source of randomness when the nonce space is exhausted.
In Simple Terms
The time recorded in a block header when the block was mined, used for difficulty adjustments.
A field in the block header recording when the block was created, expressed as Unix time (seconds since January 1, 1970). Used in difficulty adjustment calculations.