Security deposits are legally the tenants funds held in escrow by the landlord until the end of the lease. They tend to be heavily regulated in tenant law, with stipulations on how large they can be, what can be deducted from the security deposit upon lease termination, how long after lease termination the landlord has to return the remainder of the deposit, whether they have to be held in an actual escrow account[1], etc.
Prepaid rent is just that - it's rent for some future period that the landlord is just receiving early, so doesn't come with all the legal strings attached[2]. The landlord recognizes the income when they receive it and it becomes their money to do what they want with at that point.
The nuance goes beyond just semantics, too. If a tenant files for bankruptcy, the security deposit the landlord is holding in escrow is considered an asset of the tenant's estate. Whereas prepaid rent is not, unless the contract was executed super recently and the judge determines it unfairly harmed other creditors and wants to claw it back.
[2] I'm sure this isn't a universal truth - in the US alone, tenant law tends to be an overlaid mishmash of federal, state, and local regulations. So I'm sure some places attach strings to it. But many if not most places do not.
It has a similar signaling effect, but someone could prepay 6 months rent and then just stop paying. With a security deposit, you have that as a buffer until the person leaves.
Perhaps the best way to accomplish this without having the landlord perpetually hang onto large amounts of tenant cash is to have a large deposit that slowly decreases over time, as trust increases. So perhaps it would be 5 or 6 months of deposit that is used to offset every 3rd month of rent during the first year.
