An SQL (Sales Qualified Lead) is a lead the sales team has personally reviewed and confirmed as "yes, this is a real opportunity." If marketing qualifies the MQL, sales qualifies the SQL. It's the funnel stage closest to the sale.
The MQL → SQL transition is a key control point: if many MQLs come in but few become SQLs, marketing's and sales' lead criteria don't match. That's exactly where a feedback loop is needed.