A Lead is a generic descriptor for any person who has connected with your company in any way. They’ve gone out of their way to identify themselves to you and express some level of interest in what you do.
They are an individual or organization with an interest in a product or service you are selling or offering. Leads are are often seen as potential customers, that need to be nurtured to move to the next step of the sales funnel.
The most useful type of leads are qualified leads.