"Any payment you receive for the use or occupation of property" is how the IRS defines rental income.
Here are additional types of rental income:
Advanced rent payments: If your tenant pays both the first and last month's rent before moving in, then you would count both payments as income for the year in which they're received.
Portions of the security deposit that you keep: The security deposit isn't counted as rental income if it's returned in full to the tenant. If, however, you keep a portion of the deposit for any reason, like to repaint the room, then that amount counts as rental income.
Expenses your tenants pay for if they're not required to do so: If your tenant pays their electricity bill and subtracts it from their monthly rent, then the sum used to cover the bill counts as rental income.
Services rendered in place of rent payments: If your tenant provides you with services in place of rental payment, like charging you $50 for piano lessons, then that $50 counts as rental income.