A prospective customer that has been researched and vetted by both marketing and sales, indicating they are ready for a direct sales pitch or demo.
Why it matters: SQLs are the primary metric for outbound sales effectiveness. A high MQL to SQL conversion rate means marketing and sales are perfectly aligned.
In practice: An MQL is contacted by a SDR, who confirms they have budget, authority, and immediate need (BANT), officially upgrading them to an SQL.