The distinction people actually mean
A prospect is someone who might buy. A qualified lead is someone you have checked. The word doing the work is "checked", and everything else in the vocabulary is downstream of it.
The reason the terms get muddled is that most teams inherit them from a sales methodology built for a much larger organisation, where marketing and sales are separate departments who need a handover word. If you are one person answering your own enquiries, the handover does not exist and the vocabulary feels like ceremony. The underlying idea still matters: some enquiries deserve an hour of your time and some do not, and finding out which is which before you spend the hour is the whole game.
Prospect
Somebody who plausibly fits. They are in a market you serve, they look like people who have bought before, and nothing about them rules them out. Notice what is missing: any evidence they want the thing, can pay for it, or intend to decide any time soon.
A prospect is a hypothesis. Treating a list of prospects as a pipeline is how forecasts end up wrong, because a hypothesis you have not tested is worth roughly nothing.
Qualified lead
A prospect who has answered questions. They have told you what they are trying to do, roughly when, roughly what they can spend, and whether they are the one who decides. Now you have evidence rather than a guess.
Some teams split this further into marketing-qualified and sales-qualified, which is a distinction about who did the checking rather than about the person. If nobody hands anything to anybody at your size, ignore it.
Why the line matters more than the labels
The practical value is not in naming the stages, it is in deciding what each one is worth. A prospect is worth an email. A qualified lead is worth an hour. Getting that backwards is how a calendar fills up with conversations that were never going to close, which is expensive in a way that never appears in any report because the cost is your time rather than money.
Moving the check earlier is the only real lever. Qualifying on the call means you have already spent the hour by the time you learn the answer. A short set of questions in front of the booking gives you the same information for free, and the people who will not answer them were, in practice, not going to buy.
Where it becomes a link problem
For anyone whose enquiries arrive through a bio link or a campaign, qualification and attribution are the same conversation. Knowing that a lead is qualified is half of it; knowing which post, ad or link produced them is what tells you where to spend next.
That is why qualification sits next to campaign tagging rather than in a separate discipline. A qualified lead with no idea where it came from improves this month and teaches you nothing about next month.