MQL vs SQL: What's the Difference?

An MQL (Marketing Qualified Lead) has engaged enough to be considered a real prospect by marketing's definition - downloaded a guide, attended a webinar, visited high-intent pages repeatedly. It hasn't been vetted by sales yet.

An SQL (Sales Qualified Lead) has been reviewed - usually by a scoring model or a sales development rep - and confirmed to match the ideal customer profile with clear buying intent. It's ready for a direct sales conversation.

The MQL-to-SQL conversion rate is one of the most useful numbers in a funnel, because it isolates lead quality from lead volume. A campaign generating twice the MQLs but converting half as many to SQLs hasn't actually improved anything - it's just moved the same problem further down the funnel.