Security deposit
Money a landlord holds when you sign a lease, to cover damage or unpaid rent, and returns when you move out, minus allowed deductions.
Many landlords charge about one month's rent. State law often caps the amount and sets how fast it must be returned and what can be deducted.
Photograph the unit on move-in day and again when you leave, so you can show what damage was already there.
Example
Rent is $1,100 and the deposit is one month. At signing you pay $1,100 + $1,100 = $2,200 before fees and moving costs.