What do you mean by lead/leads?

Greetings Warriors,

I am working with a real estate niche site and I am quite confused with the term "lead" and I hope you could share your knowledge to me.

So what is lead?

Based on my research, I found out that the precise definition of a 'lead' tends to vary somewhat. In fact, one of the biggest stumbling blocks between sales and marketing can be the definition of what counts as a lead and what doesn't. It's important to make sure that everyone involved in lead generation is in agreement with the people responsible for following up on those leads.

Most salespeople define a lead as someone who matches the criterion the salesperson has established and who has the need, reason and/or interest in pursuing the product. However, marketing professionals tend to define a lead as anyone who might have those qualities, and believe that a lead that proves to have the desired attributes is considered to be a 'prospect.'

Here is another definition of leads:
Leads can consist of the names and addresses (or e-mail addresses) of individuals, corporations, institutions, or agencies. Lists of leads can be gathered or filtered from targeted databases such as telephone and Internet directories.

So how you define leads or lead?

What are seller leads as well?

I really need some inputs from you, seasoned Warriors.

Cheers,
G.B.