Someone who showed enough interest in your marketing (like downloading a guide) to likely become a customer.
An MQL is a "hand-raiser," not a "buyer." If you treat every MQL like a guaranteed sale, your sales process will become bloated and inefficient. The goal of an MQL is to nurture the lead until they are educated enough to care about your specific solution. If they download a guide but never open an email, they aren't an MQL—they’re just a curious bystander.