Help me out here . . . what is the downside to the term "deposit"??
The interpretation varies. Some people consider it like money against a service or product that will be given sometime in the future. If the product is not delivered or the service given, then people expect to get their deposit back.
However, when you say that you need a retainer to "reserve" the date upon signature of the agreement (I hate the word contract, also), you are actually getting money against you reserving the date. If they cancel, you've done your part in reserving the date, and your retainer is just compensation (well, not always) for having lost other opportunities.

