Parameter is a limit that affects how something can be done, and perimeter is the outline of a physical area. Both words have special meanings in math, but they take off their pocket protectors and relax their definitions when they join the rest of us.

Parameter is the fancier math and science word. In mathematics, parameter is a measurable factor in a system. Outside of math, parameter is still a factor, property, or characteristic. It can also be a boundary or a limit. Here are some examples:

That makes it the heaviest observed elementary particle yet discovered, but within the parameters set by the Standard Model. (Scientific America)

"As hard as journalism is, at least you have parameters," she said. (New York Times)

Within those parameters, Ms. Gray played with color, creating an upbeat show reflecting London's mad, mad fashion world. (New York Times)

In mathematics, the perimeter is the boundary of a geometric figure. In mainstream language, it can be the limit of any physical area, often one protected by an armed force: secure the perimeter! Here are some examples from the news:

Police set up a perimeter around the house and blocked off Avery Street for safety reasons, Sgt. Scott Custer said. (Inquiring News)

Newest study finds fuel spill's perimeter expanding. (KOB-TV)

Some linguistic snobs don't like parameter's meaning of a boundary or limit when perimeter would do. But according to Garner's Modern American Usage, the use of parameter to mean boundary is "virtually universal" except by "die-hard snoots." Unless you are writing for such an audience (a math professor, perhaps), you can use parameter to mean boundary.

Just remember: The parameters of playing hide-and-seek are not peeking while you count, and to stay within the perimeter of the yard when you hide.