When you set the boundaries of something, you delimit it. Suburban homeowners often delimit their property lines with tall privacy fences.
You can literally delimit something, the way you might delimit the boundaries of your airplane seat by planting your elbow on the armrest. There is also a figurative way to delimit, like when you delimit your summer schedule in an attempt to keep your cousin from visiting you for a month. Delimit comes from the Latin delimitare, "to mark out as a boundary."