"Multi-dimensional refers to the fact that the index used to lookup data may be comprised of several dimensions. A simple key->value lookup is one-dimensional. (row, column)->value is two-dimensional. BigTables may be multi-dimensional as the index maps from (row, column, time, column qualifier) onto a value. In particular the idea that you can index values by time is novel - so you can retrieve the previous versions of a value by timestamp if you like. This implies that no data are ever deleted from a BigTable, and this is indeed the case. Once a value is written, it’s written for life, uniquely identified by its timestamp. Well - almost. Applications may specify to BigTable that only the most recent n or only versions that were written recently should be kept, and BigTable is free to garbage collect those that are not."