Irregular income
Pay that changes in amount or timing from month to month, like gig work, freelancing, tips, commission or seasonal jobs.
With irregular income you can't count on the same paycheck each time. A budget built on your average month will come up short in slow months.
A common approach is to budget on your lowest recent month, keep extra income in a buffer account, and pay yourself a steady amount from it.
Example
If your last 6 months brought in $2,600 to $5,200, you budget essentials on $2,600 and save anything above it.