A first time deposit, or FTD, is the first money a newly registered user puts into their account. For brokers and exchanges it is the point at which a lead stops being a cost and starts being a customer.
In marketing terms, the FTD is the metric that actually matters. Optimizing campaigns to registrations is easy and misleading, because a registration that never deposits is worthless. Serious brokers measure cost per FTD, not cost per lead, because it ties spend to revenue.