In a practical sense what it really means is "non-RDBMS." Even more specifically what it means is "any of the database technologies that were created or popular

In a practical sense what it really means is "non-RDBMS." Even more specifically what it means is "any of the database technologies that were created or popularized in the period following RDBMS dominance in order to circumvent the performance limitations RDBMS technologies encountered at internet scale." SQL just, unfortunately, was treated somewhat synonymously with RDBMS.