A pebibyte is a unit of data storage that equals 2 to the 50th power, or 1,125,899,906,842,624 bytes.
While a petabyte can be estimated as 10^15 or 1,000,000,000,000,000 bytes, a pebibyte is exactly 1,125,899,906,842,624 bytes. This is to avoid the ambiguity associated with the size of petabytes. A pebibyte is 1,024 tebibytes and precedes the exbibyte unit of measurement.
For a list of other units of measurements, view this Help Center article.