Sanjoy - A lead in Eloqua is a marketing lead, and not necessarily a lead that is yet 'hot' enough to send to the CRM to have Sales look into. A lead is generated for the contact if the contact does a 'response' on a Campaign. Effective a lead is a response.
A New Lead = the number of leads that were new (i.e. response done created a new lead for the contact) for the Campaign shown, regardless of when these new leads responded to the Campaign.
These, and all of the other insight terms are defined in the Eloqua Insights Reports Details Guide http://docs.oracle.com/cloud/latest/marketingcs_gs/OMCAA/pdf/Insight_ReportDetails_Guide.pdf
Greg
ps As I believe this definitively answers your question it would be great if you marked this as the answer :-) Happy New Year